1) trichomes
2) density
3) pistil coloration
if you haven't tried different points of harvest, it'd be wise to do so. trial and error is the only way to figure out your preferred point to harvest. Even the strain will impact when it is best - not just on length but that an early or late harvest can potentially translate to different things.
some strains hit peak thc% before they max out on yield... another urban myth is that clear trichomes are devoid or low on THC. this is not true. Cloudy is first sign of degradation of the trichome.
I'd wager most of the trichomes covered up by layers of growth are amber by harvest, since they are older. The surface will not always tell the full picture, so use the other 2 factors for determining harvest date. Use all three to determine if something is "early" to "late" and the whole spectrum between those two. Each strain *could* vary.. but seems a good chunk act fairly similarly.
i've had a plant or two that never ambered up... one of those plants i kept waiting and waiting and it died on the stem, lol. So, only using 1 metric will eventually lead to a problem. This is more of an art than a science at the moment... do what you enjoy best based on the outcome. That requires a few experiences before you feel confident about anything.