Get a 30X hand-held microscope with light and check the trichomes/resin glands. It will be one of three colors: clear, creamy white, or amber. Do not harvest when they are all clear because it is too early and the thc is less psychoactive. Perfect harvesting time will depend if you have an indica dominant or sativa dominant strain. Harvesting when some of the trichomes start turning milky/creamy white will give you more of a head soaring high which is perfect for sativa dominant strains. Harvesting when half of the trichomes are creamy white and half are clear will give you a perfect balance of cerebral and body high. And harvesting when most of the trichomes are amber color will produce a physical body high which will be good for indica dominant strains. All this info I got from Jorge Cervantes' book: Marijuana Horticulture The Indoor/Outdoor Medical Grower's Bible. Hopefully this helps you.