When they crow or lay an egg. That being said, I have a black silkie roo who crowed at 3 months. He will be 5 months soon and it is obvious by his comb and little red wattles (and crowing) that he is indeed a he. I have blue silkies that I am trying to sex but am quite sure that one is a he as he also has tiny red wattles and a larger comb than the others. Its mostly a wait and see as they are notoriously difficult to sex.