get a 30-60x or better magnification an duse trichome color -- this correlates to THC potency more than any other method of choosing when to harvest.
you can read up on all the guesses as to when the best time is... in general, ther's a 2-3 day window you'll want to harvest. it happens fast when it happens, in my experience... too glassy one day, then hella smokey.