
Your best place to start is with the breed club for the bred you are considering. anyone seriously involved with their bred will belong to their breed club and will be bound by their rules and codes of ethics, and of course will wnat to have a good reputaion among their peers.
You can find contact details at the bottom of each standard -page on the Kennel club website (clisk standards, then the grup your breed is in, and then the breed, scrol to the bottom of the page and the club details are there.
You can also ask the breed clubs about any of the breders you ahve seen advertised,a dn they will be able to tell yu if they are club members with a good reputaion, or the opposite.
Genuine breders are actually delighted to have people contact them when reserching the breed, and would far rather get to know you before they have the likelidhood of puppies available.