There are several key differences -
First and foremost, Dominiques are rose-combed and Barred Rocks are single combed. Barred Rocks probably arose at least partially as mutations from Dominiques. They were all referred to at one time as Dominickers or Dominiques or Hawk-colored Fowl or Barred Rocks. But then they were classified again, Dominiques being the true first breed of America.
Dominiques also posses a curved back, from their head to their tail on the roosters should be like a soft "v" idealy, same with hens. Rocks have flat backs. Also, Dominiques are a dual-purpose breed, originally smaller then most modern day strains. Rocks are larger traditionally.
And one thing - It depends on the breeder, but typically Dominiques are bred so that their barring isn't as fine as a Rocks.
Anyway, hop that helped.
JW