you examine the trichomes with a hand held microscope. When they turn from clear to milky your ALMOST done, when they start turning amber (the total percentage is a matter of personal preference) you harvest.
Get you a pocket microscope. 12.99 radio shack.
75-80% of the hairs are red chop chop.
Use the mirco to look at the trichomes when they are milky chop is you want more of an upper type high, wait a few more days till they are amberish, for more of a couch lock type high