I'd say time to change your signature! A good way to tell it their wing feathers. Just spread their wings out and see - the ones with longer feathers are the boys. Also, go by the "girls mature faster than boys" idea. Those two techniques only work if your picking from chicks all of the same age.
I don't know much at all but ... I know some breeds are "hen feathered" ... so those breeds you can't tell? I have heard this of Seabrights. I'm not sure if there are others. I'm sure lots of people here know the answer though. I just came to look at the pic of the pretty boy.