Hey chief, here are a few good links for you to check out:

Sexing/preflowers
Sexing/identifying preflowers
Sexing/male preflowers

I like to take clones from each plant, and then flower those after they've been rooted. This way, I'm not having to stress plants that may still be slightly immature by switching the light cycle on all of them... then having to revert back to vegging later. There's another link for that under the Sexing area in the FAQs.