
All of Brainless' advice is very valid, you can also ask about Hemivertebrae, have the parents been xrayed (onlly a small proprotion of hte breed have, there is no KC scheme yet, but some research is being done), have any previous pups or relatives suffered from it (very important). Have you decided on the "type" of pug you like? There are some quite wide ranging differences in type whilst still generally meeting the breed standard. Is the Mum of your preferred type? One of my pet hang-ups is nostrils, check that the mum (and dad if he is there) have open nostrils, not pinched, and not restricted by the nose roll. Some lines seem to have picked enormous wrinkle over nostrils and breathing.
The rest is pretty much what you would look for in any litter still "in the nest". Does Mum look well, is she friendly and approachable? Are the pups plump and glossy, clean and sweet smelling, is their box/bed/puppy pen clean? Is the breeder asking you lots of relevant questions to make sure you are the right sort of puppy buyer? How experienced is the breeder? Why did they do this particular mating? Are they keeping any of the pups themselves? Are the pups KC registered? What info etc do you get with the puppy? What back up is available?
Hope this helps, good luck with the pups, it is very hard to resist a baby pug!
bye
Gwen