I once read a silkie breeder say that at shows she has seen the judge and the breeder arguing over the sex of a silkie that was MUCH older than 3 weeks old. More like months old.
So until they lay or crow you never know.
However, sometimes a pullet or cockerel will appear so "like a pullet or a cockerel" that it is usually true.
We love to guess!! BYC'ers LOVE to guess the gender!!!!!!!!!!!!
With silkies I don't hardly even look at the combs when they are young. I look for streamers, stance, expression, and roo attitude. Even then I am fooled sometimes. And then there are those that I won't even guess on because I cannot tell.