Danib, If you got the chicks straight run, odds are, you will have to wait at least until they are 6 weeks old before they start showing initial signs of gender. However, since I first made this post back in 2009, I have had few chicks I thought were girls at 6 weeks age, that turned out to be boys, and chicks I was sure were boys that turned out to be girls. About 8 weeks it is a little easier and three months, even better. If you got the chicks as sexed pullets from a feed store or hatchery, they are pretty good at sexing them and 90% should be accurately sorted.
However on a side note...My 11 year old daughter did a school science project on sorting straight run chicks for gender. She tested a bunch of the old wives tales methods. The method that was the most reliable, was laying the chick on its back in the palm of the hand. The chicks that squirmed a lot and didn't relax after a few seconds, always turned out to be cockerels. The ones that laid still, were either girls or boys, but the test identified at least half of the boys with the squirming. We have used the method on a few different breeds and it is about 75% accurate, so it is at least better than a coin flip. It isn't perfect, but worth a try. Tail feather growth is another thing I keep an eye on. Boys sometimes have slower growing feathers in some breeds. I have lots of chick picks with some details on her science project at my blog
Hanbury House, but it is just about bantam cochins, and I haven't raised any black australorps since 2010.
Best Wishes.