What a loaded question!!!
A reputable breeder is hard to come by. You really can NOT look on line and know if the breeder is reputable or not. When I'm researching breeders, I look for years (literally) to choose the right breeder.
First, let me say that I know of NO reputable breeder of any "doodle." They are a mixed breed - not a breed - and as such shouldn't be sold for more than say, $75. When you purchase a purebred from a breeder, you are paying for stable genetics. A cross such as a poodle/golden is NOT a stable gene pool, regardless of what the "breeder" may tell you. Research genetics, and you'll get the "drift." (And yes, there's a pun in there. :))
But onto a reputable breeder, the best place to start is a dog show. Head out to a local dog show and start talking to the people showing the breed(s) you are interested in. Start asking about health questions, temperament, the difference between pet quality/show quality, the breed's pros and cons, etc. Start asking about breeders - who they recommend and who they don't.
Then with that information, you go from there, doing on-line research and speaking with owners of puppies from the breeder.
A reputable breeder will do health testing on ALL pups - pet and show quality. They will have done research into the ancestors of their breeding stock. They will know the breed's genetic health issues and will know if any of those issues showed up in the ancestors of their breeding stock. They will have done DNA research. They will have done all the usual testing (hips, eyes, etc.). They will have a spay/neuter contract on pet quality dogs. Their contract will have a clause that requires you to return the pup to them if something happens to you or you have to give the dog up. This keeps their progeny out of shelters. They will be a low volume breeder - ie. only breeding a few litters a year. Breeding too many litters means the pups don't get the socialization they need early. They won't let the pups go home till 9 or 10 weeks old. Pups need this time in the litter to "learn" doggie language. They will do puppy testing (ie. Volhard testing).
And the list goes on.
I have very good friends who breed who I do not consider to be reputable breeders. A reputable breeder will focus first on temperament and secondly on form (ie conformation). These are hard to come by. Much research has to be done to find a great breeder.