It is quite difficult to test a software developer. The difficulty is strongly connected to an amount of time that is required to confirm skills and competences described in resume. This usually involves tedious tests and long discussions about theory and practical experience.
In my opinion, it is the only way to confront reality and grow as a software craftsman.
Fortunately, this can be spiced up with simple gamification like leaderboards or online contests. A wide range of tasks and an ability to compare your results with others is great opportunity to learn something new.
I think that sites like https://www.hackerrank.com/ are a great place for undergraduates and people after bootcamps to extend their knowledge and understanding.
There is a large variety of algorithmic tasks (sorting, graphs, dynamic programming, ...) and other disciplines like functional programming and machine learning. I wish that during my time at university had similar tools.
For more seasoned developers it could be a great jumpstart into new programming languages or different programming domains. I observed this in my own case, when I wanted to "sharpen" my own skills in
golang and machine learning.
I strongly encourage you to give it a try.