I have only a few dozen as reference, but after my first 7 boys, I've gotten pretty good about telling the gender of my orps.
I prefer at 2-3 weeks to watch the wattle development... nothing is foolproof of course... but I've noticed that the boys so far, have a part in the fuzz; a line of skin showing through, very clearly and perhaps even a little growth.. while the girls still look very much like chicks in the face and have little to no part in the fuzz for the wattles at that age.
but that is just so far... who knows! Not to mention sometimes, the just look bigger, act like roos and even start to crow early! LOL