Actually, let me correct that somewhat....they will pre-flower at a certain stage and some will show a few pistils in the preflower....I have never tossed a plant until after switching to 12/12 and making sure....perhaps others have a better way to tell pre-flowering males/females. I personally have made several assumptions on the sex of plants only to be found wrong later (not new). I don't wait for preflowering to turn them...just do it and cull the herd later..