First, a little basic botany. MJ will not show sex until the plant is sexually mature. This happens ~5-8 weeks from germination and evidenced by alternating nodes and pre flowers. It will show regardless of the light cycle, so it's best just to keep them on whatever you use for vegging.
If you get to the alternating nodes and still no pre flowers, here is a quick and easy trick:
Take a clone sized cutting from each plant, label carefully so you know who belongs to who. Put each cutting in a glass of water (cut the stem again underwater), then put these glasses in a 12/12 cycle/room/box/whatever.
The cuttings will survive long enough to show sex and that's all you need for them to do. They probably won't root (I've never tried and toss them after the sex is shown), but you will know the sex of the plants they came from, which is the whole point.
Then you can do whatever you have planned with the plants you now know the sex of and with no stress to them.
Wet