Recently somebody asked me about how to figure out if somebody is a good personal trainer. This is an inherently tricky thing to do and I clearly have my own preferences. Also, my advice is most useful to Americans since I know more about American certification organizations than those in other parts of the world.
First off, my bias is towards a program that is strongly grounded in modern scientific research. Further, research must be understood and evaluated for quality and relevance. The most common abuse of research comes from over interpreting or extrapolating the results of a single study. I keep my recommendations to those which have the backing of at least several studies (the exception being those blog posts which are commentary on a specific research article).
As such, I place a high value on a personal trainer who has a strong background in exercise science and who applies that knowledge to their training. This second part is important. There need not be a connection between education in a topic and using that knowledge. There are afterall MD's who will support homeopathy.
At a minimum I'd look for a personal trainer with a Bachelor of Sciences in a topic such as Exercise Science, Exercise Physiology or Kinesiology. This automatically rules out almost all people who work as personal trainers. And if you think most personal trainers aren't that useful, then perhaps you'll see my point. To put this into perspective I'll compare my education to a typical personal trainer course. For each chapter or unit in such a course, I have a 3 credit course from an accredited University. That is, for each week they've spent on a topic I have a 16 week course complete with it's own text books, assignments and tests. The difference then is quite large.
I strongly recommend a trainer with a Certified Strength and Conditioning Specialist (CSCS) cert from the National Strength and Conditioning Association (NSCA). The CSCS is the basic requirement for working as a strength coach for a professional or college sports team. To my knowledge it is the only such program in the US. This cert requires a 4-year degree like the kind I outlined above.
(Contrary to it's name the NSCA is actually an international organization with branches in several European countries, Japans and other parts of the world.)
The only other organization whose certification I trust is the American College of Sports Medicine (ACSM). However, the ACSM does not offer a sport or strength specific certification and they focus on public health and disease related physical activity programs (which is a great thing, but not necessarily as relevant to this blog).
There exist too many other certifying organizations for me to comment on all of them. I simply take an exclusive approach that a certification from another organization needs to prove itself. As personal trainer certification is in no way regulated in the United States it is not reasonable to assume that 'certified' has any meaning unless it is backed by a well regarded institution like the ACSM or NSCA.