You cannot sex them by feather at this age. They all grow in the same pattern first and then, later, molt into either male or female plummage. That said, if you got these from a hatchery bill sexing tends to be fairly accurate on hatchery strains. Those with a green tint to the bill are males, those with bright yellow/orange bills are female. The third picture down, for instance, is a male. No doubt in my mind. The top picture also looks to be male. The second picture is blurry and iffy. Possible female. Clear close ups of their heads would make it a lot easier to say.
As srpaint says however, the quack is the most accurate way to tell at this age. If you don't have clear differences in quacks, I'd say you have all males. The females QUACK and loud, the males make a raspy dack, dack sound. Pick them up and segregate them from one another and listen to their calls.