
Over time (many litters and simply being around the breed for years) you become able to recognise potential - and for sure, it's 'easier' with some breeds, than others. In Bassets, the faults are basically there early on so I've been able to 'reject' the obvious then, but as they do horrible things during growth, what you see, or don't see early on, may all change, and change back again!!! It helps to have several pairs of eyes on the subject. Perhaps the owner of the stud dog has seen what he throws in his previous litters and can help.
We are usually able to select a couple of the bitches (we rarely kept back a male, although did!) to run on - and in truth both usually stayed because by the time it was more obvious, I'd become far too attached. In one litter we had, of 9 (5m, 4b), I was all set to keep back the best two bitches, and then the owner of the sire came over to see them, and told me I'd be daft to let the best male go!!! So we kept him too. Our final decision was made as late as possible, but as said, I usually knew what wasn't going to make the grade so they went off by around 10 weeks. If, later on, I saw them again and the ugly duckling had developed into a swan I'd try to persuade the owners to show.
I honestly don't know how Terrier breeders 'choose' - perhaps it's more by markings in some breeds at the end of the day? This is something we don't need to worry overly about with Bassets much as some markings can give a false impression - white along the topline can make it look as if it's dipping for eg.