When shopping around for a personal trainer, there isn't a lot of information available to base your decision off of. The most important factors that should influence your choice is the trainer's experience and education. To become a personal trainer, you either need to have a degree or get a certification and sometimes it's both.
With so many certifications out there, what can they tell you about your trainer? First, check what certification your trainer has. You can find out by asking them, looking at their business card or asking the gym they train at.
Next, go to your favorite search engine and type that certification in to find its homepage. Finally, check what someone must go through to become certified with that company. Check how long the personal training coarse is and how much it costs. Generally, the shorter the coarse and the cheaper it is, the lower the quality of the certification.
The good certifications (ACSM among others) usually have a six week or longer coarse to go through every possible detail. Some other certifications are less than $100 and only require an easy online test that anyone can pass.
Your trainer's educational background can also help you out. If they have a four year degree in a related field such as Exercise Science, Exercise Physiology, Athletic Training, Physical Therapy, Dietetics or Food and Nutrition, they should be more than qualified to train you even without a certification. You should also ask how long they've been in the field. The longer the better. If they are bad trainers, they won't last very long.