This is just a guess, but it seems the ones that get their tail feathers early tend to be female. In your case the one you *think* is a roo has much more developed wing feathers and has sprouted a tail - when compared to the chick in the next pic, which has no tail feathers and even, short wing feathers, I'd say #2 is your roo and #1 is a pullet.
Then again, buff orps are darn near impossible to sex until they crow or lay, in my opinion. It's really hard to tell when all the feathers are the same color!!!