Do you just have the two? The one with the thicker legs is probably a roo. I heard that's the best way to sex them before they crow and before the saddle feathers come in.
I have two pullets and when they were both only a few weeks old, some people were sure the one was a roo (her comb was bigger than the other's and her comb and wattles turned more red a few weeks ago). I'm still pretty sure she's a pullet, though, because I would imagine she'd look much more roo-ish by now (and her legs were the same size as her sister's).
Anyway, I've learned with my current Littles (14 weeks old now), that relying on comb and wattle development isn't always a sure thing. The size of their legs are much more accurate. Hope this helps! And if that one does prove to be a roo, I hope you can keep him. I've had some awesome RIR roos in the past <3.