You should have a pretty good idea with the RIR's by 4 weeks. The boys telecast pretty early with larger, red combs than the girls (who can get substantial comb, but stays yellow until POL).
I've not had Australorps, but I believe them to be an average maturing bird, so again you should tell by the comb size about 4 to 6 weeks of age.
Comb is the best indicator for roo's vs. pullets. Compare chicks of the same breed rather than your RIR to your Australorp.
Roo's tend to feather in slower than the girls, the girls will get their back, wing, and tail feathers sooner than the boys...who lag behind looking quite scraggly for several more weeks.
Roo's also tend to have a cockier attitude, chest bumping and running around like crazy little birds, but a number of pullets can act pretty snotty too.
Roo's tend to have heavier legs, but some pullets I've had were Amazons in leg size too.
So overall, the best indicator is the comb size, and the best way to do that is compare chicks of the same age and breed (like siblings). Sometimes chicks purchased from the same hatchery batch can mature differently because they come from different parents, but the line generally develops similarly.
I agree post again with photos at about 4 weeks or so. Give us a good side profile (without holding the bird) and a head shot showing comb.
LofMc