Some good advice above, by and large. I'll add a simple way to start:
1. Find one who looks like you want to look. That may seem overly simplistic but truly, it helps a great deal to see that they know how to represent, and can walk the talk.
2. Ask for their client references- and ask if they have before after pics of their clients. Trainers who don't get results are called either paid friends or baby sitters
3. Make sure they have some valid certification, only because a goid trainer has a foundation in the biomechanics of exercise and nutrition and can apply it. Also make sure that they carry liability
4. Now, interview them. And ask at least these questions:
A. How do you go about setting goals for your clients?
B. Do you monitor diet and work with your clients on this?
C. Who do you have the most success with, men or women? (trainers do tend to lean toward one or the other by nature of their rapport and interest)
D. What is the best way to achieve my goals?
E. What makes you a better trainer than anyone else (the only acceptable answer is 'youll see results' or 'i can get you to where you want to be')
If they hem and haw on any answer, probe.
If they are defensive or vague, move on.