This is about Doms, but the same rules apply to BRs. There are very good pictures in the site.
http://www.dominiquechicken.com/Sexing_Dominique_chicks.html
Historical Document
Kansas Agricultural Experiment Station
SEX IDENTIFICATION IN PUREBREDS
BARRED PLYMOUTH ROCKS
It had long been recognized that the size of the light head
spots in Barred Plymouth Rocks varied in the two sexes. The
males tend to have larger headspots and the females to have...