You want to ask yourself: is this the kind of person I wouldn't mind being stuck in a foxhole with? (metaphorically, of course)

At the end of the day, you need to figure out if they are self-motivated, can take the initiative, and most importantly, can get shit done.

Unfortunately, there aren't really good substitutes for figuring those things out other than experience working together.

A "trial period" where both of you collaborate on a project has worked well for me in the past.