I hope someone will confirm this but in RIR and production reds the roosters are often smaller and have less feathering on the backs...now on young chicks there is a way to examine the vents...and since I have all pullets at 7 weeks, I haven't seen the male version...
What I did was look at a Picture of a male and female,
The male has dark brown before its bare feet, Females has light brown,
(ITs RIRs, This is at 2-4 weeks old that i did this,)
Mine are Now outside fully.
I have 24, Hopefully keep that limit for selling.