A barred or cuckoo hen under a non barred or cuckoo rooster will usually make what's referred to as black sex links. Not every rooster will work, things like dominant white can mess it up, but your red/buff/black roos will work. As stated, males will have a light spot on the top of the head, females will have a solid dark head. The males will grow up barred, the females will be solid or have leakage of varying degrees on the throat/neck/chest, depending on the roo used for the cross.
How you wrote your cross (barred rock/bcm) indicated the barred rock was the roo. If this is the case, no sex linkage will happen. Half his chicks will have barring, regardless of gender.
And, just to be picky, sex linking is different than auto sexing. The cross we're talking about here is called sex link, it's a one time deal. If you breed a sex link to a sex link you won't get another sex link.
Autosexing breeds breed true. Legbars are the one that comes to mind for me. You breed a legbar to a legbar and you get chicks that can be sexed at hatch, and they continue to breed true.
She wanted a second rooster and it looks like she got her wish 
