The best way to tell is by the trichomes on the buds themselves. your looking at about a 70% Milky to 30% clear when your flush with water should begin. At 20-30% amber trichomes on the buds is when it's the best time to harvest, remember on the bud not the sugar leaf! A good way to see these is with a Jewelers Loop, or a wifi microscope hooked to a cellphone or laptop! Hope this helps! Happy growing!