i am not an expert on sex linking, but i think that a black over a buff does not sex link. i do know that a red over a white does, and a barred over a red. however with hatchery chicks it doesnt always work due to hatcheries mixing in another breed for hybrid vigor.
lowell stromberg wrote a book on sexing chicks that describes some of the sex link characteristics, ive read it, ive practiced and im still not always getting it right.
the best method i have found that works for me, look at the size of their feet. pullets will usually have smaller toes. on a hatch of 100 birds i pick 40 with the smallest toes, and 40 with biggest toes, and scratch my head on the other 20. on the 40 i choose as pullets i normally dont have more than 2 cockerels.