My standards are:
Make sure the pups are clean, their behinds too and not swollen or red.
Their eyes clear & clean.
Check out the surroundings, that gives you the idea of the care of their dogs.
Look carefully at the parents, gives you an idea of how the pups may turn out.
There should
atleast be the mother on the grounds. If not the puppy was whelped and raised by someone else.
Look at the activity of the pup, you want a frisky, wide eye, playful, active puppy.
I don't believe in inline breeding, ask to look at the parents Pedigree.
The Breeder always gives the new owner a contract for Health (at least 10 days, most times more...) and an opportunity to get a Vet check on the puppy, with the acceptance if anything is wrong your money or an exchange will be given.
If your paying a significant amount for the puppy, the puppy should be registered with a well known club and example
AKC.
Other than that Yorkies are the best
Good luck