It varies with individual chickens, I'm sure. Before I found this site, I always just waited to see who crowed. After reading these boards I paid close attention to comb color, And by two or three weeks old I had tentatively labeled all my chicks either "hen" or "rooster" based on how red or large the combs were. Now, at eight weeks, everyone I thought was a rooster is crowing, and everyone I thought was a hen are now obvious hens. So in my limited experience, the comb color is a pretty reliable thing to look for.
If you're lucky, you could get a freebie. This year one of my little roos started crowing when he was only several days old. That was a surprise!