Are my chicks hens?

Hi,
I have 3 Pekin Bantam chicks rapidly growing and I wanted to know how I can tell if they are hens and how old do they need to be before I can tell?

They are 5 weeks old at the moment and are in there own coop with mother hen away from the other 5 hens and cockrill, when is a good age to start intriducing them to the rest of the brood and letting them live with them in their big coop?

Thank you

some breeds are very difficult to sex - I had an araucana last year that I bought as a hen but at 9 months she started to crow and tread her sisters   
Often you have to wait until they either crow or lay eggs.