Hi sandusky28,
At 3 weeks of age, you can tell with a good degree of certainty by looking at the combs and wattles. I find this method about 99% accurate with a little practice.
From above, the comb of a rooster is pinker and thicker.
The comb of a pullet is narrow and should have no pink in it yet (or almost none).
However comb sexing can be harder on breeds that are prone to early heavy development, or chicks on higher protein feed, and of course on araucanas and similar birds (though I usually see differences at exactly 3 weeks no matter what the breed).
Before this, some indications can include longer fuller tails and better feather coverage in the girls, while boys sometimes have really short tufty tails and fewer feathers. But some birds are naturally slower feathering regardless of sex, so sexing by feather growth isn't as accurate as by comb.
cheers
Erica