Can I tell?

[colby bush]

Can I tell the difference between a rooster and a hen when there 2 to 3 weeks old by the sound of the chirp?

 

[Pyxis]

Unfortunately not, although some roosters do start trying to crow very very early - I've even seen video of a 2 day old cockerel trying to crow! But generally speaking, no, you have to wait until they start developing their sexually dimorphic characteristics to tell.