I've gotten pretty good by judging the snood size when they hatch. The males will usually have a larger one than the females, although it's kind of tricky when they're still that little -- sometimes a male will be small compared to the rest of it's broodmates, so that can throw me off. For sure, I can tell by the time they're 6-8 weeks. Lagerdogger is right -- the males are usually bigger, more aggressive, and have larger snoods, where the females are more petite. To me, they just have a "girly" look to them.