Plants will certainly show signs of sex before 12/12 - preflowering. I just culled a male and am certain another is female, out of eight plants. The other six have calyxes, but not developed enough for me to determine their sex with certainty yet. I probably will know them all, one way or another, this week, and when I have culled all the ales THEN I will go to 12/12. Or maybe I'll keep vegging a bit longer while I clone the best of each type and make sure the clones are good (White Rhino and AK-48).

Here are some not-totally-reliable ways to determine sex:

1) Taller plants are often males
2) Males usually display their sex first, so an early calyx, likely male
3) If the calyx has a stalk, probably male

If you see a calyx develop into multiple banana-bunch looking pods, that's definitely a male. (This will happen regardless of photoperiod, 12/12 not needed.) If the calyx spews out some white strands, that's definitely a female. Use a 30x jewelers' loupe or something to look for calyxes. You can get a cheap microscope from Radio Shack, or a 30x loupe from ebay (that's what I got, works great).

If you don't want to wait for the calyxes to develop naturally to the extent that sex becomes obvious, you can take a cutting, plop the stalk in water, and expose it to 12 hours of light followed by 12 hours of darkness and it should flower and display sex definitively. This won't work unless the plants are mature enough to display sex though, and sex isn't determined until around the 3rd week of vegetative growth.

Final note: you desperately need more light for those plants. I mean, badly. I'm not saying you won't get anything from those plants - most likely, you will. But you really need more light. The plants aren't stretching because the lights are too far away; they're stretching because they're desperate for more light, period. And yes, they will grow significantly during the flowering phase.