Check if they are registered with the Kennel club.
Go to their premises, is it clean?. Look at the conditions any dogs or pups are kept in.
Read their conditions of sale.
It should include that you must take the pup or dog back to the breeder if at any time or for any reason you cannot keep the dog or pup.
Ask to see the pedigree details of their dogs, and details of any parents of pups, health check results.
Always see any pups with their mother, and if the Father is not there, ask if its possible to see him. (He may belong to someone else).
A good breeder may also ask the buyer of a puppy, questions they consider relevant for the pups future welfare.
GSD owner for 56 years. (UK)