"Can you describe your background and training in marital therapy?"|
If the therapist is self-taught or workshop-trained, and can't point to a significant education in this work, then consider going elsewhere.
"What is your attitude toward salvaging a trouble marriage versus helping couples break up?"
If the therapist says he or she is "neutral," or "I don't try to save marriage, I try to help people" look elsewhere. (I'd also run if the therapist says he or she does not believe in divorce.)
"What is your approach when one partner is seriously considering ending the marriage and the other wants to save it?"
If the therapist responds by focusing only on helping each person clarify their personal feelings and decisions, consider looking elsewhere.
"What percentage of your practice is marital therapy?"
Avoid therapists who mostly do individual therapy.
"Of the couples you treat, what percentage would you say work out enough of their problems to stay married with a reasonable amount of satisfaction with the relationship."
--"What percentage break up while they are seeing you?"
--"What percentage do not improve?"
--"What do you think makes the differences in these results?"
If someone says "100%" stay together, I would be concerned, and if they say that staying together is not a measure of success for them, I'd be concerned.