People don't have to take a class to train a dog. I think people put too muh emphasis on classes or titles. Just meet the dog and owner. I'm sure you'll know pretty quick what type handler/dog team they are. Meet at a park and ask the owner to do some simple commands likes come, heel, sit, stay, etc. That will tell you more than a piece of paper will, IMO.
Hit them in the pocket book with a deposit and pet rent and then ask to "interview" the dog. Most crappy dog owners wouldn't even bother to let you meet the dog.