If they health test, if they prove their dogs either in the field or in the ring (preferably both, especially for breeds like GSDs), if they provide a lifetime guarantee, if they have no history of genetic health disorders in their dogs' pedigrees, if they can provide you with lifetime support, and if they rescue or otherwise endorse rescuing, chances are you're on the right track.

However, I must say that GSDs aren't in good shape overall. Demand to see an extensive pedigree and ask the breeder to point out any dogs with poor hips or elbows or other heath disorders. Some breeders will allow their dogs to throw a few dysplastic pups and call it "flukes". I differ in opinion, and I probably wouldn't go to a breeder who allowed that. JMO.