Hmm. If you look at the hackle feathers can you see a difference? Cockerels will generally have pointed hackle feathers, hens rounder ones. I also consider which has the thickest legs, which of the same batch first shows reddening of the comb and wattles (cockerel) and whether the bird has a small tufty tail quite early on (hen) or a rounded rump for longer (cockerel - the feathers that emerge will be the sickle tail feathers).