Barred rock males are super easy to tell from females and it has nothing to do with comb or wattles. 9 out of 10 times the males are going to be much lighter than the females and more of a grey color than black. Take a look:
Like I said, its not 100 percent accurate but most of the time the coloring is going to be the number one tip off that its a roo. Otherwise you have to wait until the bird is older and the comb difference is very obvious and the crowing begins.