Timing when to harvest , if you look through a scope 30-60x, you will see clear,cloudy,or amber trichomes. which are the hairs on buds and leaves. on the tips of these hairs are little balls of resin. If they are clear,early, then you will have more of a head high, if they are more amber,later, then you get the body stoned effect.
Then again some will just go by amt of Redhairs on buds.