I always thought that purebred meant she was 100% (hence 'pure') from two dogs of the same breed. So, unless you have papers on her, I just don't think there is any way to tell just by looking. Of course, there is the obvious mixed breed, or, if you knew that this litter of pups was from a lab and a boxer. Well, then, of course, those dogs are not pure bred, and that is probably noticeable. But, to know that something is pure bred just by looking seems a little like guesswork.

Whether the breeder is good or not doesn't seem to be the indication of the dog's lineage. If she didn't come from a registered litter of pups, can you ever be certain she is pure bred?