Signs of a good trainer:
1. Has never, or rarely, been bitten.
Would you hire an electrician who bragged about how many times he has been shocked?
The vast majority of dog bites and dog aggression are fear-related. Any dog is capable of biting, given the right circumstances. Good trainers avoid putting a dog over threshold.
2. Works to change behavior, not suppress it.
Anyone can use punishment to stop a behavior-for the moment-the key question is; will the behavior be repeated? What about when you are not there to stop it? Better (and much easier) to teach your dog what TO do versus what not to do.
3. Does not use implements or methods that cause fear or pain.
This means choke collars, shock collars (sometimes called e-collars), ear pinches, leash pops (more often called the gentler sounding “corrections”) hanging, or helicoptering. This trainer also does not toss chains, water balloons, cans filled with pennies at the dog, or use blasts of canned air.
4. Has passed some measurement of skills and knowledge by an outside source.
The Certification Council for Professional Dog Trainers, (not all trainers listed with the CCPDT are positive reinforcement-only so do your homework), and trainers certified by the Karen Pryor Academy are some good examples.
Other recommended sites are:
Association of Pet Dog Trainers (not all trainers listed here are positive reinforcement-only so do your homework)
Truly Dog Friendly
International Positive Dog Training Association