With Black Australorps, it's really hard to tell at first, but as you gain experience raising them, you can start to tell at younger and younger ages. Regardless of gender, they look pretty similar. When I first began to raise them, I couldn't tell males from females until they reached about 12-14 weeks of age. Now I can tell much more accurately starting around 4 weeks old. There will be a few that are hard to tell until they are older. Once you've been raising them for a while, you start to recognize the subtle differences between both behavior and appearance. The males are more aggressive. They are more likely to spar with each other at a young age. Their comb also begins to develop more quickly. Often the ability to tell them apart is somewhat intuitive. You may not be able to exactly put your finger on how you are telling the difference, but when you look at one and take into account how he looks and behaves, you are fairly certain he is male. That said, there are still some slower-to-mature males that are really hard to distinguish from the females until they are older.