If you want to sex them, but maintain the vegetative growth phase you should do it via cloning. What you do is take a single clone/cutting from each plant. Keep the clones under 24hr florescent lighting for two weeks, so that they establish roots. Then, change the light cycle to 12/12. After another two weeks, your clones will show their sex and you'll then know which plants are males and which are females. The whole time you are doing this with the clones, the original plants can remain on their normal vegetative light cycle continuing to grow. Once you sex the clones, get rid of the plants that correspond to the male clones.