Ask what certifications they have. Then research those certifications. It has recently come to light that some breeder certifications are virtually meaningless. For dog breeds prone to hip dysplasia, ask if the parents of the pups have been certified by the Orthopedic Foundation for Animals (OFA). Research the breed you want to buy to see what other certifications or tests are recommended then make sure the breeder has done those tests.
Be sure to visit the breeder's site and look at the place where the dogs stay. Are they in a home or facility? Is the location nice and clean? Do the dogs get regular interaction with people? If a breeder doesn't want you to meet that their facility, this is a VERY BAD sign and most likely because the animals are raised in what society calls a puppy mill.
Also try to meet both the mother and the father of the pups. Are they generally friendly? Do they appear happy and healthy? Check their eyes, nose, ears, and body. Look for parasites. Ask if they've been treated for worms. Most puppies will naturally have worms so don't be concerned if they do have them. Just make sure the breeder is handling it properly.
Two important things you should know: 1) The easier it is to buy the puppy, the more likely the breeder is a puppy mill breeder or a backyard breeder. A backyard breeder is an ordinary person who breeds their dog to make money but either doesn't know how to properly perform the proper genetic breeding practices to reduce the chances of inherited breed disorders, or doesn't care. A reputable breeder will ask you a lot of questions to make sure you will be a suitable owner for their dogs and will want to educate you about the breed to make sure you are prepared. They will also gladly answer any questions you have and not be evasive about the answers. 2) The cheaper the puppy is, the more likely the breeder is a puppy mill or backyard breeder. A reputable breeder charges more because they have done more research in breeding quality dogs and because they run more health tests and maintain more certifications.
If you want a good healthy dog and don't want to support those terrible puppy mills, do your research and buy from quality breeders only (or adopt from an animal shelter or breed rescue group).