Sometimes it's pretty hard, especially as chicks or young birds. It's where you have to get more familiar with body types and shapes, not just colors. Leg color, comb type and body type are all taken into consideration. Plus, how common is a breed? Where did they come from? Most hatcheries offer only certain breeds, so you can sometimes rule them out simply by availability.
White rock has a straight comb. Wyandotte has a rose comb. Both have yellow legs. Wyandottes I believe should be more rounded on the lower part of the body, I've always heard "shaped like a gravy bowl".
Buff rocks aren't too common, so it can be an easier one to remove from consideration based on availability alone. Rocks have straight combs and yellow legs, Orpingtons have straight combs and white/pink legs. Orpingtons are also larger, very rounded and fluffy while Rocks have normal feathering, not the fluffballs.