Most trainers offer a complimentary consultation and / or assessment for a chance for you to meet with them and go over your health history, goals and possibly perform a fitness test to set some baseline measures. During this initial consultation it is a chance for you to determine if this is the right person for you. Interview them as they are interviewing you. Below are some questions you may want to ask:
Did you go to college? If so, was it in an exercise related field? If not, did you do an internship before training? If they did go to college in an exercise related field, most programs require internships as part of graduation requirements. If they did not go to college or do an internship, how did they get experience?
Do you hold any certificates? If so, what are they? The two most reliable in the industry in my opinion are the NSCA and the ACSM. NSCA or National Strength and Conditioning Association which has two certificates for personal trainers, the CSCS or Certified Strength and Conditioning Specialist and the CPT or Certified Personal Trainer. What sets the CSCS apart from the two and all other certificates as far as I know is that you must have a bachelor's degree in order to sit for it. The ACSM or American College of Sports Medicine is considered the bible for exercise prescription in which most universities use for teaching their curriculum. There are several programs of specialization but geared more for clinical settings and special populations. However, they have the HFI or Health Fitness Instructor which is a great certification for personal training as well. NASM or National Academy of Sports Medicine has also made a name for itself. A few other certificates you may see are ISSA, ACE and AFFA. These others are not as reliable as NSCA or ACSM but they are still reputable companies. The only downside of certificates is that it is just that a certification. A person can have all the knowledge in the world, but without any hands on experience, you will not need to know how to apply it. This is where experience is important. If they did not do an internship and do not have a great certification, how long then have they been training? If they have not worked for more than two years in the field or along someone reliable … then stear clear!
Ask them for any testimonials? Do they have results or any before and after pictures of people they have worked with? It's always important to hear from others. Unless you were referred to the trainer by a friend or colleague I would be asking for these.
Ask them what they are passionate about? Sometimes you can tell if the person is genuinely interested in helping people or if they just want to make money. Do they want to get to know you and ask questions to let you talk the whole time, or are they only talking about themselves?
There are many inexperienced trainers out there charging way too much with a lack of knowledge and experience. I want you to be educated in your decision prior to investing your time and money on a trainer.