Make sure both parents have any relevant health tests for their breed.
You can check if a dog/bitch really have had the health tests they are telling you they have, on KC website.
Avoid crosses, especially "designer" crosses - major puppy farm and backyard breeder money makers really love these.
If you still really want a cross breed make sure parents still have relevant health tests for their breeds.
Look for a KC Assured Breeder - I don't think its a completely foolproof scheme but it certainly provides a much, much greater chance of the breeder being genuine.
Does the breeder use a contract? Will they take pup back at any point in its life? Does the contract say you can have a full refund if a vet finds anything wrong with pup in first 48hrs you own pup?
Meet their dogs in general. See how they behave in the house - are they relaxed around ordinary household noises (suggesting they do actually live inside the house normally)? Are they friendly, healthy, etc?
See pup for first time whilst still feeding from Mom, as she'll obviously have milk - so you know its not just a "show bitch" they've put pups with to sell them. Then you can go back again once they are older for a "proper" interacting with pups visit.
Don't expect pup to be cheap but also don't think that paying more means a better raised pup either.
Avoid any puppies labelled as imperial - done to sound posh/exclusive and allow dodgy breeders/dealers charge more for the runts of litters.
Don't expect to just go out and get an 8week old pup - good breeders will have had pups reserved way before this time.
Don't use preloved, gumtree, etc. Find puppies through KC Assured breeders or through a breed club.