Here is what you should minimally expect from a qualified and caring personal fitness trainer:
Evaluation: Your first session should consist of a lot of questions and discussion with very basic exercises that will help your trainer to see what areas you need to focus on. The first session is usually not the best, as it is more diagnostic than anything. It should set the tone for your future progression of sessions, which should kick your butt and help you achieve your goals.
Professionalism: Clear communication, clear billing and charging, timeliness, cleanliness, focus, and attention to detail are all crucial qualities in a productive trainer/client relationship. If you have a regular trainer who has never really corrected your form or discussed why you are doing the exercises that you are doing, that is a warning sign.
Good Results: After a few weeks do you feel or see any difference in your body? Are you in less back, neck, or joint pain? Are activities easier to accomplish? Have you lost or gained any weight? Do your clothes fit better? If not, you need to get a new trainer. And if things are worse-if you are still in pain and not getting stronger-this is another warning sign.
There are many talented fitness professionals out there, so never feel like you must continue to pay and suffer through sub-par fitness training. It really will only hurt you in the long run.