While some of us try to, most of us do not have "perfect dogs". (I know I don't!) All you can do is be informed so any decision you make is a sound one. Especially regarding a dog that will hopefully be with you its entire life! Here are some pointers that may help you distinguish a breeder you may want to avoid. No one thing makes someone a less than reputible breeder, these are just things to watch out for.
1) If the parents of the puppy you are looking into buying have names that don't include a kennel name, for example "Baileys Martini", chances are that breeder has something to hide and doesn't want the pup to be able to be traced back to them. BEWARE!
2) If the breeder is unfamiliar with terms like OFA (Orthapedic Foundation for Animals) or OVC (Ontario Veterinary College) or Penn Hip, or says things like "my dogs are VERY well bred so you needn't worry about their hips or elbows" or "I've never had a problem "...BEWARE!
3) If a breeder offers to drop the price for you to get you to take a pup... you get what you pay for!...BEWARE!
4) If the breeder will not talk with you at length about health issues, and talk about what health issues you could come up against with a puppy, even discussing problems with the dogs they already own...BEWARE!