really depends on the breed. there is the size of the comb and wattles, the color of comb and wattles, the over all color of the breed, and the age.
on a cockerel the comb and wattles are usually bigger and redder than a pullets, most breeds the cockerel will start to feather in in a different color around 13-15 weeks give or take a few weeks. the age of the bird also helps with gender. you can look at my albums there should be some good shots in there but they aren't labeled pullet or cockerel. if you look in the cochin one there is a partridge one (pullet) and a mottled one (cockerel). my d'uccle album has a pair of golden necks in it as well.
both genders have saddle and hackle feathers, on a cockerel these will be long, slender, and pointed and come in around 13+ weeks, while a pullets will be fatter, shorter, and rounder.
here's what those feathers look like on a cockerel/rooster