When I was looking for the place I asked all my local horse forums to recommend me trainers. And not just recommend, but tell what horse was in training, why, and what was the end result. Also visiting a facility and a lesson/training session tells you A LOT. Usually you can tell right away if the trainer is indeed good with horses. I drove through every single one on my list, looking at the horses there, conditions, food(!), and talking to the trainer in person and asking uncomfortable
questions. I also ALWAYS google the name whether it's a new trainer, vet, or farrier I wanna use. You can't believe how much info you can find (sometime good, sometime not so) about most people these days.