The cocker spaniel breed club has some good information on health testing here. Preferably you want both parents to have had a BVA eye test and a BVA gonioscopy, the former within the twelve months prior to the mating and the latter within three years prior. Both should also have been DNA tested for Progressive Retinal Atrophy (prcd-PRA), Familial Nephropathy (FN), Adult Onset Neuropathy (AN/AON) and, if you’re looking at working types, Acral Mutilation Syndrome (AMS). Hip scoring is both mentioned by the breed club and recommended by the KC but it seems there are very, very few breeders who actually do it.
You also want to check the inbreeding coefficient of any litters you’re interested in, this article explains what the COI is and why it’s important. A very simple explanation is that whilst health screening schemes help breeders reduce or eliminate the risk of puppies being affected by certain specific conditions, keeping inbreeding levels low reduces the chances of puppies being affected by either untestable or as yet unknown conditions (there’s a list on this page of health issues seen in the breed which can’t be tested for but likely have some degree of heritability). I would personally be aiming to try and find a litter with a COI below 5%.
You can use the KC’s Mate Select tools to check both health test results of the parents and the COI of the litter providing you have the registered names of both parents. I believe all the aforementioned tests are recommended by the KC so if the parents have been tested the results should be there.
The Champdogs website has quite a good little guide to buying a puppy plus an accompanying list of questions to ask breeders. It’s also worth reading their guide for breeders interviewing prospective buyers as it’ll give you an idea of the sorts of things breeders should be asking you.
Lastly this site has a breakdown of the developmental stages of puppies and gives you an idea of the basic things a breeder should be doing to maximise the chances of their pups being prepared for their new homes and growing up into well rounded adult dogs. It’s also important to acknowledge that how a dog is raised is only part of it, temperament is also the result of a dog’s breeding. Parents with good, sound temperaments are more likely to produce pups with the same. This article (written by a veterinary behaviourist) is well worth a read.
In an ideal world you’d have no problems finding a breeder who does everything perfectly however, realistically, you might have to end up compromising somewhere, even if it’s only travelling further than you’d prefer or waiting longer for the right pup. All you can do is research as much as possible and decide what you want from a breeder.