What I've seen suggested elsewhere is having publicly viewable code on github or the like - that would effectively be your coding resume and evidence of your ability.
True but it is not that hard for someone to fake that if they really want to. That said, faking history in github likely takes enough technical ability that the person could probably do a decent job programming if they use their powers for good.
Just ask them simple questions about their code. "Why did you use a class here? What does this function do?". Or even what motivated them to do the project or what the hardest challenges were. It should be pretty clear whether they're the real deal or not.