Will have the parents tested for genetic hereditary diseases and will provide certificates to show the parents are free of these diseases. Canine hip displasia is one very common hereditary problem, large dogs absolutely need certification before breeding. Many countries make it illegal to breed dogs without testing for genetic defects, but not so in Canada and the United States. This is why you need a responsible breeder;
Has done extensive genetic testing on the breeding stock used and is willing and able to discuss whatever faults his/her dogs have (and all dogs have them. No dog is perfect.);
Will offer a health guarantee;
Will offer a genetic disease guarantee;
Will take back the dog at any time during the dog's life if you become unwilling or unable to care for it.
Will be available to you for advice during the life of the dog.
Is knowledgeable about his/her breed and breed standard;
Is active in the breed in some fashion, perhaps in the conformation ring, or in the performance aspect, such as obedience, agility, hunting, earthdog, schutzhund, ring sport, fly ball, etc; Thus can get you involved more in your dog.
Can discuss the socialization of the puppies and can make recommendations for the continued socialization and training;
Remember this will be your companion for the next 10-15 years. Although a reputable breeder sells his dogs for more money because is costs more to breed good puppies, you will have a much better companion and most likely spend less in vet bills.
