> But can you really tell if a pup is going to be so far from breed standard not to be shown at such an early age ?
It depends. Lots of faults, such as mismarks, are seen very early on. Others, like incorrect bites may be apparent early or gradulaly develop into an undesirable state. Tailsets are seen fairly early, but tail carriage may not be apparent until later. Shoulder placement, toplines, angulation etc also varies - basically if it is poor early on it is unlikley to get better, but sometimes promising pups can "go off" later. This is why it is easier to guarantee that a puppy will not be show standard than it is to guarantee that one will be

Then you have movement and attitude. Most baby pups move nicely, as they mature and the angles change movement can "go off", attitude is usually apparent early on, and quieter pup is unlikely to set the show ring alight. Of course you also have the upbrining and training which factors in before you get the finished article - usually out of the breeders control after a pup is sold.
We often run on several pups from a litter before we decide who we are keeping, the changes from 8 weeks to 6 months can be amazing, but not everyone has the facilities/ability to do this. Another thing to bear in mind is a persons opinion of what is a show dog, different people have different standards, different abilities to asses pups and dogs. Then, when one refers to a show dog, are we referring to a dog who has a great chance of becoming a Ch, or one who will be lots of fun to show at Open shows, doing plenty of winning but be unplaced or get low places at Ch shows. Then, how qualified is the breeder to evaluate the pups, what success have they had? Have they years of experience and lots of success with well established lines and an indepth knowledge of how their pusp grow on, or are they newish to breeding so have not yet had the opportunity to make an educated guess at the pup's potential (this later classification often consider they have a whole litter of show quality pups - an experienced breeder is delighted to think they have one or two in a litter who may make it!)