Choosing a therapist is a tough decision, but sometimes you feel you have to do it. How to choose a therapist is another story.
If you have the ability to be picky, it's probably best to go to a professional association whose beliefs are not anethma to your own, and ask for referrals in your area. If you choose just any therapist, it's possible they don't believe in multiiples. If you choose someone who belongs to a professional organization dedicated to education and research on multiples, at least you're unlikely to end up with a member of the False Memory Syndrome Foundation (FMSF) -- a group who deny the existence of multiplicity, childhood abuse, ritual abuse, and similar on the claim that therapists hypnotize their patients and thus it's all made up.
Your first time in the office, you're there to interview the therapist. Make sure you bring some questions with you. If you're going to tell the person that you're multiple right off the bat, you ought to ask something like "How do you feel about integration? Is that a suitable goal for treatment?" And make sure you agree with their stance on that pivotal issue.
If you're going to spend time feeling the person out and not come out as multiple right away, you may have other questions, as to length of the sessions, and questions about whatever your excuse is for being there -- say it's anxiety issues, you could ask: "For every 10 patients you have that seem to have an anxiety issue, how many have you put on medication?" "How would you treat anxiety without medication?"
Look around the therapist's workplace. Do you feel comfortable there? Listen to all the instincts/voices, and pay attention to your anxiety level -- if the pit of your stomach is responsive to questions (some people's are) you can play litmus test or twenty questions? with it: are you just anxious because you're meeting someone for the first time? Or is it the environment? Do you think you can work with this person? Do you like them? What's your initial impression?
If something about them makes you uncomfortable right away -- whether it's the color of their nailpolish or hair, or you can't put a finger on it, you might want to find someone else. You really need to feel comfortable with the therapist and you might miss clues consciously that other people in your head are trying to give to you.
Also, you probably don't want a therapist that you could be sexually attracted to. Keep that in mind.