Remember, a good protection dog should be a freindly happy obeident member of the family that is also an alert and couragious protector. The difference between a good trainer and a great one is simple. The great trainer loves his ART, and is a perfectionist, has a broad background of experience and has worked many many dogs. They should be willing to spend the extra time needed to get the dog and handler to work as a team. Also be on the lookout for that really well behaved dog at a public park, whose handler looks like a fluid extension of the dog.
Word of mouth is a powerful tool.
A trainer should also offer private training first, then a group environment to keep your skill sharp. If you find a trainer you are considering, invite yourself to the trainer's group class and watch. Talk to the clients, watch the interaction between the trainer, dogs, and clients. It should be a fun yet focused environment. You must meet with the trainer in an almost personal atmosphere, as you must feel comfortable with the trainer.