Is the breeder genuinely caring? How does the breeder interact with the puppies? The most important thing is that the puppies have the best start to life.
Are you happy with where the puppies are being kept? Is it clean and do they look well cared for? Does the place smell unpleasant? Is there enough light? Have the puppies enough room to move around easily? Are you allowed to see where they are actually kept or does the breeder insist on bringing them to you? If so, why? Do they have fresh water?
Are the puppies outgoing or cowering in a corner? If they are cowering it could be the sign of a nervous dog or one that has not been treated properly by the breeder or been socialised.
If they are asleep are they contented? They should be plump but not with distended tummies which might be a sign of worms.
Do the puppies (and their mother) look healthy? They should be bright and lively (unless asleep of course). Do they look well fed?
Are they scratching a lot? If so it might be a sign of fleas or possibly something more serious.
Do they have runny noses or eyes? These are not signs of healthy puppies.
If there are other dogs around do they look well cared for?
Do the breeders have lots of different breeds and seem to be continually breeding?
What is your overall impression? If you feel comfortable then that is probably a good sign. If you feel uncomfortable for any reason, make your excuses and leave. Don’t waste your time or, even worse, buy a puppy because you feel sorry for it.