I cross reference breeders on Champ Dogs and the Kennel Club.
You should speak to several breeders before you pick one, and then I always ask the following questions on the phone when I talk to them:
How many bitches do they have and how many litters will each bitch have (more than two litters per bitch is, IMO, a big red flag and more than one a year from multiple dogs is a huge red flag because it is SO exhausting to breed a dog that no one has the energy to do it properly more than once a year. And, of course, if they have multiple breeding bitches then chances are they aren't looking after the puppies or the bitches properly. A good breeder will sleep with the pups for all eight weeks AND be there constantly during the day, cleaning up and socialising them. It's a 24/7 job - no one can reasonably do that more than once a year unless they are superwoman/man
How old is the bitch (KC say one year old, I think two - minimum - before they can be bred).
Why did they choose the sire they did (don't be surprised if you can't see the sire. Most breeders don't own the sire these days). Is it because of good hip/eye scores etc? Is it because of his appearance? The COI?
What are mum AND dad's scores for the key health tests (you can see the recommended ones on the KC and Champ Dogs)
What is the inbreeding coefficient of both parents (KC website will tell you breed average. But don't be wedded to this. My Golden is 10% inbred, which is higher than average, but his hip and elbow scores are incredible. BUT too high and it does mean they are potentially breeding two dogs that share a grandparent, which is unacceptable when it is a very popular breed.
How long have they been breeding? (Not a top priority, but the length of time is very telling. Anyone who has only been breeding for a few years is likely trying to capitalise on post-COVID puppy booming IMO).
Then you'll need to decide which breeder you think ticks the most boxes for you and you feel comfortable buying from.
When you go to view a litter:
Ask to see evidence of the health tests (a good breeder will proactive show you)
Ask to see all the puppies (if they say no, then run. Just because some are sold, it does not matter. You should be able to see all puppies interacting with their littermates).
How are they socialising the puppies (hoover, people, children, different types of surface such as grass, concrete etc)
Will there be a contract of sale? (There should be one)
Can you see the family tree (this isn't hugely important, but its good to be sure the breeder has been careful to avoid breeding from similar lines)
Will puppies be wormed and vaccinated
Will the puppy be microchipped
Can you meet the puppy several times
Will you pick the puppy or will you be matched
If you do view a litter and something doesn't feel right...don't be afraid to walk away. I have done. Many times. It's no bad thing. You have to be SURE you are buying a dog from a good home and that it's been loved and cared for. KC does not necessarily mean it's a healthy or well tempered dog (look at what they've done to French Bulldogs and GSDs).
Edit: I should say, for my most recent dog. I had to visit twice (including with both my children), do video tour of my home, show proof of ownership of my house, explain my experience with the breed, and she wanted to see our finances (nothing to invasive - just proof we could actually afford a puppy) before the breeder would confirm she'd sell me a puppy. So do not be surprised if you are thoroughly vetted. A good breeder should want to meet you several times (and IMO do a video tour) before they sell you a dog.