If you purchased them through a hatchery, most hatcheries tell you to expect a 90% accuracy with sexing. Depending on how many you ordered, you could get a roo or two in the bunch, or you may be lucky enough to have just all pullets.
If you purchased through a feed store, I hear they are sometimes less accurate with their sexing, so you could get a few roos there too, or not.
Otherwise, I would just watch for any changes in one of them that you dont see in the others, like a larger, more red comb when the others have smaller, more yellow combs.