I am a terrible judge of "the quack", particularly early due to late bloomers, but by 10-12 weeks or so, a combination of "the quack" and the drake feather are reliable indicators for me. I put less stock on behaviors, as I've seen hens seemingly "mount" or bite the back of the neck of other hens to show dominance early, and I've seen drakes absolutely savage new drakes in much the same way, for much the same reasons - in fact, I have three of my pekins separated out for just that reason right now. Too small to defend themselves successfully, too big to keep in a brooder. Have them in a 4 x 12 (roughly) grow out pen in the barn, giving them an outdoor run (weather permitting) today.