Some breeds. Depends on hatchery/breeder stock. They MUST use feather sexing genes if this is to be relied upon 100%. I don't think there is any way to tell at this age what gender those chicks are, unless somehow by their coloring. They are definitely EEs - a buff would have solid coloring (see buff orpington chicks). The parents may have been a buff and something else. I have one that was *advertised* as a "buff" - he is most definitely a black breasted red, first and foremost!
Some breeds feather out faster if, they are pullets. I've bought straight run chicks and been able to get just what I wanted by looking at wing and tail feathers.

