Ky, an old hippy woman showed me the best way to sex plants in veg. No stress to the plants at all. It does take attention twice a day for a couple of weeks to work though.
You mark a limb on each plant with a bread tie or something easy to see. After 12 hours of light, you cover the tip of the marked limb with a small paper bag. {like the ones a beer fits in} Once your lights are off, remove the bag. Repeat this everyday and you should see flowers on the marked tips in 10-14 days.
Hope this helps.