Here is a link to a group called the Association of Pet Dog Trainers. There isn't much a requirement to join this group, but most people who do use mostly positive reinforcement training techniques…or use the least aversive methods first. Anyone listed there who has the letters CPDT after their name had to complete a set of fairly rigorous requirements to attain that title...so they would have some proof that they are qualified...not to say that the other people aren't these people have just gone to the extent of making it formal.
I hope this link works:
I am working with an APDT trainer, they are very expensive if you have them come to your house one on one, at least this one is. She is very good, but it is hard to get my girl who was abused to allow the woman inside. I have stopped having her here for the time being, and may pursue classes when I get my girl socialized better! Carol