You asked how I know:
The Barred Rock is a cockerel color, much lighter than a pullet would be, which is the way barred varieties are, plus his legs are almost clean yellow. Pullets have some dark wash down the leg fronts most all the time. At that young age, pullets of that breed do not have huge red combs like that, either. Barred Rocks are my favorite of all so I've hatched and grown out literally hundreds of them.
The black one, probably an Australorp, has a pointy, shiny fringe of saddle feathers on the back, hanging over the side, just in front of the tail, as only males do.
Below are two of my BR hens when they were 12 weeks old--you see how dark they are and how small the combs are, not to mention, the leg color?

Edited by speckledhen - 6/16/12 at 4:03pm