Help needed sexing rir chicks

[Tj11]

Hi all I've got 30 rir chooks ranging from 5weeks old to 5 months and I really need to try and cut back my numbers before winter ( I'm in Australia)we have an auction locally in a couple of weeks and I'd like to get rid of some chooks there but I don't want to sell all hens and be left with only roosters. If anyone has any advice on what to look for specifically in rir would be greatly appreciated as at the moment I can only see one of the older babies that I think is a rooster judging by body shape but 1 out of 12 just doesn't seem right.