Well, right now it is mainly guessing, but there are a few differences. Roosters at this age usually are developing larger combs and wattles, or at least reddening in the face. They'll also often have smaller, more ragged crests (in terms of the crested breeds), have thicker legs, and be in general more gawky in appearance. The bird in the second photo shows a red face/comb, a more tufty crest, thicker legs, and a more gawky appearance than the top bird, so my conclusion is that it is a cockerel. In contrast, the top bird has a nice rounded crest, round, fully body shape, thinner legs, and very little color in the comb, making it more likely to be a pullet.How can you tell? I'm lost!