This is actually really easy imo. 1. For people who have worked on open source - just look through their code, their commit and you'll see how they think and o

This is actually really easy imo. 1. For people who have worked on open source - just look through their code, their commit and you'll see how they think and operate. 2. If 1 isn't applicable, give them homework, not a test. Give them a very simple but very well documented task. Something along the lines of an authentication system, with password reset which is time restricted, basic encryption and security and that's it. This can be easily achieved in just about any language in several hundred lines of code. But given the adequate amount of time to think it through and develop it, you'll see if they come up with clever solutions to simple problems or a pile of duct tape hacks.