Do the interview in person on the premises, rather than on the phone because you'll be able to meet the breeder and observe all the dogs and puppies. The premises should be clean and smell good.
These are some of the questions you'll want to ask the breeder:
When was the mother last bred? How many litters has the mother had total? Specific characteristics of the breed. Any genetic problems in the bloodlines?
Pug - Dog Lover