***ing Rhode Island Reds.

I recently purchased 25 Rhode Island Red chicks.  They are about 3
weeks now, and growing rapidly.  What do I start to watch for to
determine ***?  It appears that some have distinctly darker heads than
others.  Will the males turn darker?  And once I can tell the males
from the females should I keep them separated?  What age do people
usually cull the males?

thanks


Hal