It is easiest, if you're inexperienced, at 4-6 months because the bucks will start "dropping".
However, I'm able to sex them for the first time at 3 weeks now and then verify around 6-8 weeks so you should be able to tell now.
If you gently pull back on their area, you will see a slit on the does. Or, if there is kind of a protruding circle, it is a buck. Usually at this age, my bucks won't extend their penis so you most likely won't see that quite yet. You just have to judge by the shape of the opening.