never heard of pre flowering..I have heard of plants auto flowering while still in the veg cycle. The only way I kno of is to set the plants to flower. Set their light to 12/12 and wait for signs of sex which should be present in a week or there after. Pluck out the males and continue ur grow. Hope this helps. Good Luck on your grow.
the best way to find sex is to flower it, I do this by cutting some clones, wait until they are strong/mature enough to flower and I throw them into 12/12 until they show. then I adjust my garden by ridding it of all males.