In most breeds the pullets feather up faster - even in so called non fast feathering breeds.
So at 2-3 days old you can see a difference in wing feathers - pullets have alternately long and short flight feathers, cockerels have shorter, much the same length flights. Then, when they're feathering up (3-6 weeks) the easiest thing to spot is growing their tails - pullets longer and sooner.
When quite a bit older (6-10 weeks, how many weeks depends on breed) you can start to see more significant wattles and combs on the cockerels. And eventually notice that the end of the cape feathers are pointed on the cockerels and rounder on the pullets.