In hatchery birds, the only difference between the two breeds is the comb really. They have both been crossed back and forth and are for the most part the same size and color, just have different combs. So if you got it from a hatchery and it has a rose comb it would easily qualify as a Dominique.
But the Heritage/Exhibition strains of the breeds have quite a bit of contrast as the Dominiues are supposed to be a bit slimmer than the barred rocks and the color and stance seems to be a little different but I dont remember what the tail angles and such are supposed to be for the two. But there is considerable difference in the REAL examples of these breeds.