Some of the suggestions listed above are great. Between jeanne and Euryleia's posts any sub is off to a great start in getting to know a prospective Dom.

I'll add two things to this thread - not specific questions to ask, but a couple things to consider as you're asking. First, ask for clarification on anything that could possibly have more than one meaning. If a Dom describes himself as "poly" for example, ask him to explain to you what that means to him. Does it mean he'll expect you to interact with his other subs, or that he'll have separate relationships with multiple subs? If the answer is the latter, will he tell you about those other relationships? Most of us have a working definition of what BDSM terms mean to us, but don't expect that to be the universal connotation.

Second, if someone is of unscrupulous character to begin with then there's no reason why they would suddenly become upfront just because we subs are asking the right questions. Regardless of how you want to define "rape," for example, I simply don't see anyone that has ever participated in it saying, "sure, I've gone past limits and forced myself on a sub before." Maybe there's an exception of honor among rapists out there, but I wouldn't' base a relationship or even my basic safety on that assumption. Long winded story short - pay attention not only to what is said, but to actions you can confirm and that the prospective Dom can back up. Pay attention to what remains unsaid - questions he dodges or seems uncomfortable with and so on.