In general, roos will have thicker legs and bigger combs than hens the same age and breed. There are differences in feathering as well. You might want to post some pics over in the "What Breed or Gender" section so people can help you out with that.
How old were they when you bought them? If chicks, they should be about 21 weeks if I'm counting correctly. Pullets mature at different rates - some may not start laying until 24 weeks or so. Has it been very hot where you live? Some chickens are more affected by heat than other and may be not...