if you look at the nodes of the plant (where the branches go out at 90 and 45 degree angles from the main stock) you will see these small little triangular flap looking things ^ sort of like that. They will either sprout white hairs. you will know what these are when they appear. They are white hairs, usually 2 at first at the highest or second highest node of the plant (when they first form). These are considered Female.
Males will appear with small spherical O or Q looking things. Same place, but no hairs what so ever. They are very easy to tell apart. If you have a bulge, you are not a girl.