Should you look at their academic qualifications, number of courses attended, degrees achieved, Personal Training Certifications and number of years in the industry? Should you look at how they looK - If they have the type of body you want they must be good, right? Or, should you go with 'word of mouth' ans see who is the most popular or the most expensive, they must be good right?
If you do, then you are searching by the personal trainer that has the best marketing skills.
There are basically, two camps of personal trainers, the academic scholars who will baffle you with big scientific words that you are sure not to know what they mean and there are those with real world experience who have trained successful champions. Actually, there are many more types than that out there but for now, let's just go with these two types.
The commercial role of a personal trainer is to gerate income through attracting more clients and a higher fee per client. It is not to actually do what is best for the client and train them the quickest way possible. No, that would bring them less revenue. They need to find clients and keep them dependant on their personal training service.