I've never had to sex them. I have bred them for fishing bait, and it has been their business to decide on their gender.

The flies that walk on your butter- after walking on the dogshit outside- are fertile egglayers. Could be hermaphrodite or female, I wouldn't know.

The maggots in Peace's photo are still on the food. They look like blue-bottle maggots. Housefly maggots are skinnier.

'The Life of the Fly' by Fabre will tell you all you want to know.