Here's a loaded question(a good one, by the way)....anyone can hang out a shingle and call themselves a horse trainer.....
When I'm looking for a trainer, I'm looking for the number of years involved in the horse world, who this person apprenticed under, his success in the show pen, what his specialty is, how he treats his clients and their horses and his credentials must be impeccable. Has the trainer received national accolades from his breed organization? And, does he hold a judges card, if so, from what organization.
I'll be watching and looking at him or her a little more closely.
Word GETS amongst horse people and if a trainer is inadequate you'll hear about it soon enough. If the criticism comes from a fellow trainer in confidence, you bet I will pay attention. I pretty much know who the top trainers are and their respective specialties here on the West Coast as I'm at many shows and keep an eye and ear out for whose having a successful year in the pen. I also subscribe to several journals and keep track of trainers that way.
This is my opinion on what makes a trainer...(many different influences make a successful trainer)......