A fancy way of doing sexing early is to take a small clone from the mother plant (one you have) setup a micro mini grow room, we're talking 1 cheap 28w CFL no fans, nothing, just a hole for air, and set the timer for the CFL on 12/12 - If you see balls on the clone, it's a male, if you see pistils, it's a female.
This way you can tell if your mother plant is a M or F by sexing the clone as it has the same DNA as the mother plant. This also allows you to stop wasting electricity and your own resources if its a male, an added bonus is that if the clone is a female, it will also allow you to lengthen your vegging time to get more colas and higher yield.