Perhaps more important than the questions, are the answers, and specifically, the consistancy of the answers.

It's easy to tell the truth, over and over and over and over again. It's harder to be consistent if one is even stretching the truth, because fibs, lies, prevarications, and even exagerations, are always tailored to the immediate goals of the teller of the tales... and keeping all the stories straight, is difficult.

So no matter what questions you ask, in the course of continuing conversations, if you're paying attention, and you see inconsistancies, you're probably not talking to someone worthy of your trust.