To me the only meaningful mark of a "good breeder" is whether they have done MRI scans, have had a clear cardio auscultation on each breeding dog within 12 months of breeding, and follow the breeding protocols for both MVD and SM (which in the case of MVD means knowing the grandparents on all sides remained heart clear at age 5); eye tests and ideally also hip scores (the average score for the cavalier breed is double the average score of some giant breeds for hip dysplasia -- this IS a significant issue in the breed whatever some breeders may say).
And the buyer needs to have SEEN all these certs and discussed the breeding programme with the breeder.
Everything else is utterly peripheral. I would not even consider a breeder who doesn't do the above, as the most basic starting point.
Then after that, I would start to filter for how they care for their dogs, the number of litters they produce, whether they are even in a breed club (sadly 'good standing' can be meaningless, given the way most clubs approach health). Some health-focused breeders who refuse to join breed clubs, put breed club breeders to shame. They are VERY rare, I hasten to add, and but there are a few.
It is very tough out there to find breeders who care enough to properly test rather than offer excuses about cost, distance to scanning centres, distorting research results, outright lying, etc.
In this breed, with these widespread and serious health problems now well known, any breeder who offers these excuses simply should not be in the breed. If they are breeding they are doing it 1) for their own show egos; 2) the money. They are not doing anything for the welfare of the breed or the individual puppies they produce. End of story.