Sexing Coturnix Quail - male or female?

Usually by about 6 weeks old it is very easy to tell if your Coturnix quail are hens or roosters by the feathering.

This guy is a rooster.  His breast feathers are rust colored like a robin.

And this is a little lady.  Her breast feathers are speckled an tan colored.

By 7 or 8 weeks old the roosters will start to crow.  Unfortunately, this is also about the time they will start to fight so the earlier you can sex the quail the better.  Separating the males off into a "roosters only" pen will help reduce the fighting until you can decide which ones to keep for breeding and which will go to the freezer.