On our "About Breeders" page, we have a list of Tip Offs that a puppy seller should be avoided.
What we have listed are the most common but we want to add one more VERY important tip off:
Reputable breeders understand canine behavior and development, they do not separate pups from their mothers at an early age. Puppies need litter mates and mom to develop normally physically and behaviorally until they are at least 7 weeks of age. Staying with mom longer is even better.
DO NOT BUY from a breeder who wants to sell a pup to you at 5 or 6 weeks of age. All that means is they are probably tired of cleaning up after the litter or they don't know any better. Either way, walk away. It is not in the puppy's best interest and therefore not in your best interest to purchase a pup whose socialization has been stunted.
For more information, including signs of a reputable breeder, check out PupQuest's About Breeders page.