A good way to tell from afar is to just see the pistils. Are there a lot of fresh new white ones, or are they mostly brown with little to no new pistil growth (the white hairs). If mostly brown it's time to move to the next step which is a 60x magnifying glass to inspect the trichomes for their colour. You want 70% cloudy at least, 20% Amber, 10% clear. Then chop em. Too many ambers and the plant has gone too far and THCa degredation is taking place.
Hope this helps!