Tracy has a good point. When someone raises one breed in particular for awhile, he/she just gets a sense of who is who. I'm fairly new to Delawares myself, but I do think I have a good number of pullets out of this fourteen. Generally, males have more color at the shoulder area than the females, so that's one way to guage the sex.
Hey, Tracy, what's your verdict on the chick in the first picture? That's the one I marked with green permanent marker.