Its hard for me to recommend people because its personal choice, what I usually tell people is to watch how that person interacts with their own horse away from clinics, demos, the public and observe them when they don't think they are being observed and watch.
I have to confess and I feel bad in doing so for my ponys sake that one trainer I had I was worried to leave them alone with my pony, that wasn't a pleasant thought to go through my head. I want a trainer who I can trust with my pony to correct kindly if needs be, but whom I don't worry what they are doing to them when I am out of sight. My pony voted with its feet as well and didn't want to be left with them either. I got a new trainer pdq!
That's what led me to the above about the watching. I know people who watch me handle my cob and think I am too soft, or they don't agree with what I do or how I do, but they trust me to interact with theirs as I would handle theirs with the same level of kindness as I do my own.