trichomes
pistils
density
use more than one metric to keep you honest. At some point we all seen a plant that never turns amber... sometime you gotta use other cues. If you haven't, do experiement with early-mid-late harvests.. .you may find your preferences vary from vastly unscientific peanut gallery preferences.
For example, over-ripened buds make me sleepy AF shortly after smoking.. even before the high dissipates. Someone else may like that, but i do not. Therefore, i harvest at 10% amber or thereabouts. More than a few people might react in an angry, insecure way about this, lol. Don't let the plebs warp you... there's just a boatload of urban myth infused into put-grower culture. Even after tests totally debunk the given reasons for flushing at end of harvest, a good chunk still cling to it in a religious-like fashion. Things that get repeated for decades that are wrong take a long time to correct. people really don't like facts that contradict long-held beliefs.