I think it is on a 'trust' basis, on the assumption that if you want to do a DNA test in the first place you are honest! Not suer I'd be that sure but that's the way the KC works I think. But if they are asking fro your vet, they might ask for a second swab from them if anything turns up.
For example, you can register a litter on-line or on paper and either way the KC trusts you to show the correct parents for the pups, with the proviso that a signature is required from the stud dog owner.