1. "What kind of references do you have?" This is probably the best way to get honest information. A good trainer should be more than happy to give you a short list of previous or current clients that you can contact. Ask the references if the trainer helped them achieve their goals, how s/he helped them, what they liked about the trainer, etc. You should also ask the references the kinds of goals they had to find out if they had similar needs to yours. If you're trying to loose 55 pounds and the references only needed to loose 5, maybe that trainer isn't for you. If your potential trainer doesn't want to give you references, consider looking elsewhere.
2. "What qualifications and experience do you have?" Your trainer should be certified to help you achieve your weight loss of fitness goals. The more qualifications your trainer has, the better. But remember that there are many excellent trainers out there who will be able to help you achieve your goals even if they don't have a bachelor's or master's degree in fitness. A good trainer should have a combination of some type of qualification and some decent experience. And be sure that your trainer is certified in CPR and First Aid.
3. "What is your philosophy or approach towards exercise?" A great personal trainer should be able to explain his/her philosophy of exercise. S/he should be able to explain how s/he will help you reach your goals. How will s/he motivate you? Is there some kind of assessment process? Ask for specifics and clarification if you don't understand something. Above all, make sure that you are confident that your trainer knows what s/he is talking about.
4. "How does s/he look?" This is an important question to ask yourself. Yes, you've always heard not to judge a book by its cover. But the physique of your personal trainer will tell you something about his/her experience and knowledge of fitness and weight loss. In a nut shell- if your personal trainer looks like s/he could do with loosing 30 pounds, you might want to look somewhere else. But a person with a flawless looking body may not know the first thing about teaching you how to reach your weight loss and fitness goals. Just be careful.
5. "Am I comfortable with him/her?" This is an important question to ask yourself. Be sure that the trainer you choose is a trainer you'll feel comfortable with in vicinity to yourself, even on your worst days. If you don't feel comfortable with him/her you're less likely to accept his/her advice and guidance.

Good luck with your hunt for a personal trainer. With any luck, you'll find the perfect trainer for you who will help you achieve all of your weight loss and fitness goals and reveal a new, more fit and confident you!