I'm also going to copy and paste some posts I've done in the past because I'm lazy
Apologies though as it's going to be a bit of an essay!
First off I'd recommend having a good read through the Puppy Contract site, it covers the basics pretty well.
Breed clubs can be a good starting point but that does depend somewhat on the breed in question. Whilst some clubs are very open and forwards thinking others can be rather economical with the truth when it comes to things like health issues and inbreeding levels within the breed.
Beyond the basics a lot comes down to personal preference, the exact specifics of what makes someone a good breeder will vary from person to person. The best thing you can do is arm yourself with as much information as possible and decide what it is you want in a breeder then look for someone who meets your requirements.
The Champdogs site has some quite good, concise information that might be useful. Their Guide to Buying a Puppy and accompanying list of questions to ask are both good. I'd also recommend reading their guide to interviewing potential puppy buyers, although it's aimed at breeders it gives you a good idea of the level of interest the breeder should be taking in you.
The Puppy Plan is another website worth a read through. Lots of advice on finding a breeder/buying a puppy recommends making sure the breeder is socialising them properly, that site will give you an idea of the basics the breeder should be doing.
Health testing is important and breeders absolutely should be making appropriate use of available tests and screening schemes (this site is good for finding out what tests are available for different breeds and the KC also lists recommended tests under 'Health Information' on their page for each breed), however it's not the be-all and end-all of producing healthy puppies. Nor is it a reliable indicator of how good a breeder is or how healthy overall the dogs they produce are.
Particularly with pedigree dogs genetic diversity (or lack of it) should also be a major concern, both of breeders and puppy buyers. Some breeds are in a much worse state than others but regardless of breed it's something breeders should be thinking about every bit as much as health testing, if not more so in some breeds.
The Institute of Canine Biology blog has lots of excellent posts but I'd particularly recommend the following three:
*Understanding the Coefficient of Inbreeding
*Why DNA Tests Won't Make Dogs Healthier
*Let's Kill the Breeder Myths!