You're on the right track about setting up appointments where you can watch the trainer. Just judge how the other horses look such as condition and even attitude. You might even want to visit your top two choices multiple times. Talk the other customers or people you see while you are at the barn.
Once you think you know which one you are going with, drop in unexpectedly, or an hour earlier than your appointment ;)
Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world. ~Harriet Tubman