White Rocks have a Single Comb..
Rhode Island White only come rose comb..
If your bird has a rose comb it could also be a White Wyandotte...
Now if it has a single comb it could be a White Rock or a White Orpington and then you will have to look at the legs and see if the Yellow or Pinkish/White if there Yellow then they are White Rocks and if they are Pinkish White then they are White Orpingtons....