Don't bet your farm on that. Nor on the claim that the ones that have a distinct filled in lighter colored V on their heads are pullets and are cockerels if all dark. Got one of each last year and they were both girls.
There are plenty of "you can sex by" claims for a lot of chicken breeds. Most are based on hearsay not fact. SOME can be feather sexed. Some are autosexing/sexlinked and can be determined by color or markings. I believe all can be vent sexed though it takes a well trained person.
