My understanding is the black sexlinks are rhode island red on barred rocks ; and red sl are rir on white rir. No leghorn as far as I have researched the sexlinks. Feathersite has a great deal of detail on this.
Sex links are easier to pick by color, but even then sometimes mother nature goofs ( I have sucha goof) and humans can make mistakes on vent sexing but no such mistakes were made on my pullet order from MM this year.
Check for wing feather development; the girls grow wing feathers faster than the boys of the same breed usually. GL