Guinea Fowl are impossible to sex as young birds. I find the only way is to wait until they start calling at about 9 weeks. The call is a sure way of telling the Guinea Fowl apart as only the females have a two-note sound best described as “Pot-rack, Pot-rack”. The male birds produce a single note, be careful though because females can also produce a single call!
I've had much amusement sitting beside the pen waiting for the birds to call and when they did, marking them with the end of a stick dipped in food colouring. You could guarantee that the moment I sat down beside the pen there was not a sound and they all looked at me as if to say, “what are you waiting for?”
When the birds are a bit older the males can be identified by their larger
wattles, and bigger helmets on their head.