RIR's generally carry the slow feathering gene, so that makes them very easy to sex from about 2-3 days old. Here is a picture of mine at 3 weeks or there abouts. Notice the chick that has no feathers on back and shorter wing feathers. That's the boy. The others are female. Can you remember if your birds looked like this at around the same age. It's one of the easiest of chicks to tell the sex