There is only one sure fire way to pick the best in the litter & that is keep the whole litter.
Like said above if out of a well bred litter, your odds are going to be better.
I have always picked the average size of the litter. My buddy has always picked the biggest out of the litter & over the years I can see where his dogs have gotten bigger then mine.
I have also kept runts & without fail, they have caught up with the others & made good dogs.
I have also picked the sorriest dog out of the litter.lol
