A tech startup spent three months and £50,000 building their dream food delivery app, only to discover their lead developer had copied most of the code from Stack Overflow without understanding how it worked. When users started reporting crashes during peak dinner hours, the developer couldn't fix the problems—because he didn't actually know what the code did. The app never launched, and the startup folded within six months.
This scenario plays out more often than you'd think in the app development world. Hiring the wrong developer can cost you serious money, time, and worst of all, your reputation. But here's the thing—most companies are absolutely terrible at technical vetting when they're hiring app developers. They get impressed by fancy portfolios or smooth talking, but they don't dig deep enough to see if someone can actually code under pressure.
The best developers aren't just code writers; they're problem solvers who can think their way through challenges and build apps that won't break when real users start using them.
That's why technical tests for app developers are so important. You need to know if someone can handle the real challenges of mobile development—not just copy and paste code from tutorials. Over the years, we've refined our technical assessment process down to five core tests that reveal whether a developer has the skills to build robust, scalable mobile apps. These aren't trick questions or impossible puzzles; they're practical tests that mirror the actual work developers do every day. Let's walk through each one so you can avoid hiring disasters and find developers who'll actually deliver.
Understanding Code Quality Assessment
Code quality assessment is probably the most telling test you can give any app developer—and I mean that quite literally. You see, anyone can copy and paste code from Stack Overflow or GitHub, but writing clean, maintainable code that actually works well? That's where the real skill shows through. When you're hiring a developer, you need to know they can write code that won't fall apart the moment someone else looks at it.
Good code isn't just about making things work; it's about making them work reliably, efficiently, and in a way that other developers can understand and modify later. Think of it like handwriting—you might be able to read your own messy scrawl, but if you need someone else to read it, clarity becomes much more important. The same applies to code.
What Makes Code Quality Matter
Poor code quality leads to bugs, security vulnerabilities, and apps that crash when users need them most. More importantly for your budget, bad code is expensive to fix and nearly impossible to scale. I've seen projects where technical debt accumulates to the point that developers had to completely rewrite entire sections because the original code was so poorly structured.
Testing Code Quality Effectively
The best way to assess code quality is through practical coding challenges that mirror real-world problems. Give candidates a small project—perhaps building a simple feature or fixing a bug in existing code. Look for clear variable names, proper commenting, logical structure, and adherence to best practices. Don't just check if their solution works; examine how they've built it and whether you'd be comfortable having your team maintain it long-term.
Testing Problem-Solving Skills
When you're hiring app developers, checking if they can actually solve problems is just as important as knowing whether they can write clean code. I've worked with developers who could recite programming languages backwards but fell apart the moment they faced an unexpected bug or tricky feature request.
The best way to test problem-solving skills is through practical coding challenges that mirror real-world scenarios. Give candidates a broken piece of code and ask them to fix it, or present them with a feature requirement that has multiple solutions. Watch how they approach the problem—do they ask clarifying questions? Do they break down complex issues into smaller parts? Do they consider edge cases?
What Makes a Good Problem-Solving Test
Your technical tests should include time-pressured scenarios where developers need to debug issues or optimise performance. I like to give candidates problems that don't have obvious solutions; this reveals how they think when they can't just Google the answer. Pay attention to their reasoning process, not just the final solution.
Give candidates a real bug from your codebase (anonymised, of course) and see how they approach fixing it. This gives you insight into how they'll handle actual work challenges.
Red Flags During Problem-Solving Tests
Watch out for developers who immediately jump to coding without understanding the problem first, or those who give up quickly when their first approach doesn't work. The best developers will talk through their thought process, consider multiple approaches, and aren't afraid to admit when they're stuck. They'll also ask questions about user requirements and technical constraints before diving in.
Evaluating System Design Knowledge
When you're hiring an app developer, one of the trickiest things to test is whether they understand how different parts of a system work together. Think of it like this—anyone can write code that works on their laptop, but can they build something that thousands of people will use at the same time without it falling apart?
System design knowledge separates the developers who can build toy projects from those who can create real-world applications. This isn't about knowing fancy programming languages or the latest frameworks; it's about understanding how data flows through an app, how to handle lots of users, and what happens when things go wrong.
Testing Scalability Understanding
A good test question might be: "How would you design a messaging app that could handle a million users?" The answer shouldn't be a detailed technical blueprint—that would take hours to create. Instead, you want to see if they think about the right problems. Do they mention splitting the work across multiple servers? Do they worry about storing all those messages? Do they think about what happens when the app becomes popular in different countries?
Real-World Problem Solving
The best developers I work with don't just solve problems; they anticipate them. They know that apps crash, internet connections drop, and users do unexpected things. When testing system design knowledge, listen for whether candidates think about these real-world challenges—not just the happy path where everything works perfectly.
You don't need a developer who can architect the next Facebook, but you do need someone who understands that building apps isn't just about making features work; it's about creating stellar apps that deliver exceptional user experiences for real people in real situations.
Checking Database and API Expertise
Here's where things get interesting—and where many developers stumble. Database and API expertise isn't just about knowing how to write a query or call an endpoint; it's about understanding how data flows through your app and how to handle it efficiently. When you're vetting app developers through technical tests, this area separates the beginners from the professionals.
Start with database fundamentals. Can they design a proper database schema? Do they understand indexing, relationships, and data normalisation? More importantly, can they write efficient queries that won't slow down your app when you have thousands of users? I've seen too many apps crash and burn because someone wrote a query that worked fine with 10 records but fell apart with 10,000.
API Integration Skills
API testing is where you'll discover whether your developer truly understands modern app development. They should know RESTful principles, handle authentication properly, and manage error responses gracefully. Ask them to integrate with a real API during the technical assessment—not just talk about it theoretically.
The best developers don't just make APIs work; they make them work reliably under pressure and handle failures elegantly
Performance and Security Awareness
Pay attention to how they approach caching, data synchronisation, and offline functionality. Do they consider what happens when the network connection drops? Can they implement proper data validation and sanitisation? These aren't glamorous topics, but they're what keep your app running smoothly and your users' data safe. A developer who understands how to prioritise and protect your most valuable data will save you countless headaches down the road.
Assessing Mobile-Specific Technical Skills
Mobile development isn't just about knowing how to code—it's about understanding the unique challenges that come with building apps for phones and tablets. I've seen plenty of talented web developers struggle when they first move into mobile because the rules are completely different.
The biggest difference? Resources are limited. Your app needs to work smoothly on devices with varying amounts of memory, processing power, and battery life. A developer who doesn't understand memory management will create apps that crash or drain batteries faster than users can say "uninstall."
Platform-Specific Knowledge
Each mobile platform has its own way of doing things. iOS developers should know about view controllers, delegates, and the app lifecycle. Android developers need to understand activities, fragments, and how Android handles background processes. Cross-platform developers using React Native or Flutter should know the limitations and benefits of their chosen framework.
Don't forget about device-specific features either. Can your developer integrate with the camera, GPS, push notifications, or biometric authentication? These popular app features aren't nice-to-have skills anymore—they're expected.
Performance Optimisation Skills
Mobile users are ruthless. If your app takes more than a few seconds to load, they're gone. Your developer should know how to optimise images, handle network requests efficiently, and manage data storage. They should understand concepts like lazy loading, caching, and how to minimise the app's file size.
Testing these skills doesn't require complex scenarios. Ask candidates to explain how they'd handle a slow-loading image gallery or what they'd do if an app was using too much battery. Their answers will tell you whether they truly understand modern mobile development practices or if they're just winging it.
- Memory management and performance optimisation
- Platform-specific UI guidelines and components
- Device feature integration (camera, GPS, notifications)
- App store submission and approval processes
- Mobile security best practices
Conclusion
Getting your technical tests right when hiring app developers isn't just about weeding out the weak candidates—it's about finding the people who'll actually build something that works. I've seen too many projects fail because someone skipped the proper vetting process and ended up with developers who talked a good game but couldn't deliver when it mattered.
The five areas we've covered give you a solid foundation for developer assessment. Code quality shows you how they think; problem-solving reveals how they handle pressure; system design tells you if they can build something that scales; database and API knowledge proves they understand the technical backbone; and mobile-specific skills separate the app developers from the web developers who think it's all the same thing (spoiler alert: it's not).
Here's what I've learned after years of hiring app developers: there's no single test that'll tell you everything you need to know. You need all of these pieces working together. A developer might ace the coding challenge but fall apart when you ask them about database optimisation. Another might design beautiful systems on paper but write code that makes you want to cry.
The best approach is to use these technical tests as your starting point, not your finish line. They'll help you identify candidates worth your time, but don't forget that building great apps requires more than just technical skills. Communication, creativity, and the ability to work with your team matter just as much. Get the technical vetting right first, then you can focus on finding developers who'll fit your project and your people.
