You may have missed the RJ article on this very topic:
However, for me, it's about choosing a trainer that believes in what they teach. And their appearance is half the game. If they're out of shape, walk the other way.
Next, is about knowledge. Do they have the necessary training and credentials to be in the profession? Can they approach a session with a thoughtful routine that achieves results. Do they keep up their knowledge on the subject with additional education?
Last, is about chemistry. If you don't like being around them, you won't work out (I actually had one of these). However, if they can motivate you, your workouts will achieve results.