Mainegrown is kinda right, some strains will show sex early, while still in veg, some might not, and yeah, it takes about 2 weeks or so of 12/12 to find out. This is where cloning comes into play. If you have to space/supplies, you clone each of your plants, get roots, and throw them into 12/12 while the mother of the clones continues to veg and get bigger/heal, then you know which plants are females and which are males.