This is line dependant in the Barred Rocks. The line I had it never worked out much better than flipping a coin. They were fantastic birds all grown up, but telling them appart at chicks was impossible. They could have any amount of white or darkening on the legs and be either sex.
I think at one time it was bred for, the ability to sex them as chicks, and we have not been keeping it up since vent sexing has taken over.
Just bought four more barred rocks for my flock just found out today the lady sexs them at 1 to 3 days old. She said the females are darker then the males even that young. She looks at the little feathers under the wings I guess the darker the chicks feathers its a female. So when I got home I looked at my own that I had bought from a chicken fair 5 weeks ago and sure enough the darker ones turned out to be females and the lighter ones are male.