In my limited experience I can usually tell by 3-5 weeks. These are 4-week old Bantam chicks from TSC.
Roo:


Pullets:

The pictures aren't the best quality but you can see his comb development vs. the females'. Hope this helps.
Roo:
Pullets:
The pictures aren't the best quality but you can see his comb development vs. the females'. Hope this helps.