I would use the therapy session to interview the therapist as much as the therapist interviewing you. If you are going to have a therapist you need to have one that you are a good fit with.

A therapist probably won't be able to officially give you a diagnosis, but I know that before I could see a psychiatrist I had to be "examined" by a therapist first. That therapist only saw me once and referred me to a psychiatrist, who eventually talked me into seeing a therapist for regular visits and recommended an AWESOME therapist to me, and she has help me out so much.