One of the things I've done is to hand a candidate some existing code and ask, "What do you think of this code? Do you see any obvious problems? Are there things that work but could be done better? Is there anything exceptional in there? Tell me what you see." This allows you to test that they know how to read code, which is what they'll be spending the majority of their time doing. You can ask them how they'd improve it, which should give you an idea of how they write code, too. And of course, you can see their experience by checking out their resume and if possible running some of the apps they worked on. And if they have any code on GitHub, etc., you can check that out, too.