My 2 boys are from the same parents, different litters, about a year apart. Both of my boys are significantly larger than their littermates. They are 9 pounds, taller and longer, than their about 5 pound littermates.
Max's coat is darker, thicker, and silkier. Maybe a 'harder' silk? Teddy's hair texture is a little different, silky but different. When we are out walking though, many people ask if they are twins.
I'm not sure about telling differences at 2 weeks because weights and coats can be deceiving throughout puppyhood. As Ann indicated, good breeders can discern subtle differences and make pretty good guesses. Little things like the length of a paw can point to adult size. We knew Max was going to be on the larger side.