sexing help?

i currently have started my 2010 outdoor season. i want to keep m plants under as less stress as possible. my plants are now about a month give or take i have been trying to sex them out almost all of my plants are at their 4th to 5th node and i still am having a hard time telling. should i put them under 12\12 until i can tell and then re-veg them or woulg this create that much stress that i could fuck em up or should i just keep them on a 18\6 schedule as they are these need to go out in a month or so any help would be great.