I'm a doula as well, and I find most of the questions that you find online aren't really the best to find the "right" doula for you.
Here are my questions that I'd ask/like to be asked:
Describe your best doula experience. How did the birth go, what happened, how did the mom cope?
(This gives you a good idea as to what the doula really considers the "gold standard" of birth. Often doulas will just agree that whatever you want is all good, but they may not...