I think you know a trainer is right for your horse if you see positive results. During/after my first lesson with my trainer, my horse was completely different. He taught me how to achieve exactly what I wanted in minutes because he's got a lot of experience. Usually the first thing he told me to do worked. I lessoned with him for a year before he ever rode my horse. Got right on and had him doing everything I could get him to do and more ten times better. So, sent him for training for about 10 days and he came back even more amazing. I didn't think twice because I'd already been taking lessons from him for a year, watched him ride and train other people's horses, saw the care, their barn, all that so I trusted him and it was great. :) Before lessoning with my trainer, I took lessons from a girl and we improved a lot, too, but it wasn't anything like the leaps and bounds I made with my trainer so I stuck with him and still am.