Usually it would be very apparent by 5 months (more like 5-6 weeks) if you have any cockerels or not just by comb and wattle development.
Cockerels mature faster than pullets and would have been trying to mate them before they started to lay.
Well, now I am confused because i was sure that I would see a lot more difference by this time than I am. I am not seeing much if any difference at 5 months between any of my Australorps to distinguish the boys from the girls. I must be one of the few people on the site that I see is actually hoping to have some roosters... and if the hatchery screwed up and gave me 5 pullets instead of cockerels, that would be very, very rare indeed, too.
Could you post a few photos of some you think are boys and some that you are pretty sure are girls. What we need are shots of the head showing comb and wattles. A view showing general posture, legs, and tail can be very helpful. Five months may be too early for saddle and hackle feathers to be evident but eventually the boys saddle and hackle feathers will be longer and pointed while the girl’s will be shorter and rounded.
By this age the cockerels should be noticeably larger too.
I once got seven pullets on a straight run order of six chicks from Cackle. Even the packing peanut was a pullet. I wonder if the hatchery made a mistake with the order and sent all pullets. The odds of getting seven pullets straight run are less than 1 in a 100. I'd have been happy with two pullets and the rest cockerels. I was disappointed none were cockerels so you are not alone ion wanting a male. Luckily a few of the Speckled Sussex were cockerels so I had a rooster anyway.