You are very right to be concerned about finding a humane trainer. Anyone can call themselves a trainer and the abusive ones keep the abuse well hidden.
We recently had a "trainer" leave our barn and she went on to a new one where no one knows her reputation. This is what she does that I consider abuse and immorality:
-Her bridles were hung on what many of us called-"the wall of shame, the wall of pain" high port bits on every one, some made the horses mouths bleed.
-Constant, never ending spurring with roweled spurs
-Constant, never ending jerking on the reins-even when the horse was standing still.
-whipping the horse non stop with the reins for the slightest infraction
-something she called "detention" she brings a sweaty horse in from work and ties it in the corner of her stall for 3-8 hours (with the saddle still on) when she says it misbehaved. If she is really mad, she would go into the stall and beat them with a manure fork.
-Friday night (before the show)knee & hock injections on all of the horses to hide the fact that they are now permanently lame.
-Riding the kid's horses into the ground while they are at school so that they can come out later and ride a "dead broke" horse.
-Always telling the owner that problems were the horse's fault and offering to sell their horse (for a commission) and then find them a new one (for a commission)
About half of the owners didn't know this was going on and the other half didn't care as long as they could show. 6 of these horses are for sale for thousands of dollars but are horribly broken down physically and mentally. They are drugged when anyone comes to look.
Beware the trainer who doesn't want you around when they are working with your horse. Watch your horse's behavior when that trainer is around and working them. Keep your ears open-many people in the barn know what's going on. Insist on a list of previous clients and promise confidentiality when you talk to them. Take a good hard look at the equipment the trainer is using on your horse. Its' a scary world out there when it comes to "trainers" and you are absolutely right to be cautious.