Sheesh! I've been training for almost 15 years, and blogging for over 3 and I haven't talked about how to choose a trainer, yet? Well here are my 10 cents.
Is she/he certified by a nationally accredited organization? Since anyone can call himself a trainer with just a click of a button and a credit card, make sure your trainer is actually certified. Here is a comparison of the top certifications that actually have some clout. A reputable certification helps insure your trainer has more than just "a love of working out" as the base of their business. They are required to study, pass exams and complete continuing education courses.
Can she help you help yourself? Ask up front if she will provide you with a detailed program for you to do at home, on the road or solo. Physical therapists, doctors, and counselors all provide patients/clients with "homework". Your trainer should too. A good trainer wants to educate you so can sustain your healthy lifestyle habits on your own for the long term.
Watch her in action. Check how she works with clients. Personalized programs should vary from person to person, based on goals, fitness levels and health issues. As a side note: most gym memberships include a free equipment orientation, so personally I'd want more than a machine-based workout from my trainer. Machines are designed so folks with limited fitness knowledge can get a fairly safe workout. Your trainer should provide you with more bang for the buck.
Does he include an assessment in your initial sessions? With baseline assessments--strength, flexibility and cardiovascular health-- you'll have something to measure progress. Generally I include postural assessment too.
Do they sell supplements? I'm certified by the American Council on Exercise, and unless we're licensed/certified nutritionists we are prohibited from selling supplements or prescribing diets. Be wary of trainers who do. Often it is just a way to increase profits. As a trainer and Lifestyle/Weight Management Coach I can review your diet, make suggestions and help you recognize healthier choices. Personally, I think a diet of real foods should suffice for the average person making lifestyle changes. Trainers often team up third party suppliers of supplements, as a way to get around this issue. Once again, it's an extra revenue stream for them.
Do you like her and your workouts? In the end it all comes down to how you two jive. If she's educates you, you feel good, have fun and are motivated, then you've probably made a good match! Enjoy!