Basically at about 8 weeks old, what you see in the puppy is proportionally what you'll see in the adult. So you look for the same things you'd look for when judging a dog in the ring - squareness, angles, ratios, etc. Honestly in this litter they're all very similar, so it's a little bit of nit-picking - in my last litters there were obvious picks and obvious pet puppies - this one is much more difficult! (As much as I hate when people say it, they could all be show prospects with the right owners - Peanut is the only one who really probably is not, and that's mostly because of her tail.) Gloria is the stand out but she's not light-years better than the others; she's just a little bit better in a few areas, which makes her the best overall - more pleasing head and expression, shorter hocks, better turn of stifle, better shoulder angles, more balanced - but again, it's nit-picking as none of them have 'ugly' heads, long hocks, straight angles, etc.
