Orps are really hard to sex until at least 12 weeks old (at least for me). The Buffs are harder than the blues and blacks - Im managing them better.
Firstly Im looking at the colour of the face....in males it reddens up quicker than females. Then Im looking at feather shape. Pointed feathers (obvious around collar and on back) for males, rounded tips for females.
Apart from this its a waiting game. Im never sure till around 12 weeks for blues and blacks, but it can be 18 weeks for the Buffs (or untilone crows)