If you can, then try to get someone by word of mouth who knows about your breed. We went to training when the boys were pups paid 90 quid for each dog up front and the classes were totally unsuitable for our dogs, although other folk with different breeds did okay. I would suggest that you look for someone who says they are a behaviourist rather than a trainer..this advice passed on from the best bloke i have ever seen with our dogs who now owns a posh kitchen installation firm but used to have GSD's, and compete in obedience with them as well as train dogs for a living.
The trouble with "dog whisperer" is that anyone can call themselves one, it means nothing.
The other bit of good advice that I received, but after we had shelled out for the training we went to, is to ask if you can go and observe a class without your dog before signing up. If i had done that we would never have signed up for that particular class :-(