Obviously checking their example work is important, but in addition I like to ask them how they built the app they're showing as an example. If they can answer confidently and clearly then they probably did build it.

Great developers are concise and logical. Bad developers are disorganized and illogical.

You can't really tell a good developer from Kait talking to them though. I would recommend breaking the project down into 5-10 "sprints" where they have something to show you at each step. Ask them to set the time frame for each sprint and if they meet each sprint deadline on time and have the majority of the work to show at each milestone point then you're good to go.

Test whatever you can each step of the way so that you can get what you need functionally.