Have become a huge fan of @mattmochary 's idea of checking your energy level before and after chatting with someone, when deciding whether to work with them.
It seems a bit silly when you first hear about it, but over time it's become my #1 most important hiring heuristic.
@mckaywrigley I have a bunch of questions I ask. What would you build for yourself? What did previous companies you've worked for need? What are you an expert in? What have you noticed recently that seemed broken? If you were going to build something just for fun, what would it be?