i think you may be confused. Preflowers occur after a plant has been in vegg for a certain amount of time, usually 2-3 months(depends of strain). The plant will show either pistils or pollen sacs which will determine sex. You cant preflower a plant. The plant does that on its own. You can clone for sex. You take a cutting and put it on a 12/12 lighting schedule. this will induce flowering and tell you the sex of the mother plant. hope this helps you