Sexing chicks/pullets

So I'm new to this chicken thing, and I seem to be doing okay so far ... I got some chicks at about 3 weeks old in June, and they're now growing like weeds and fully feathered. My question is, how do I tell the hens from Roos? Some are bigger than others, but they were hatched on a hen so they're not all exactly the same age.

Help?

Thanks in advance

They are Coronation Sussex if it matters ...