Outdoors, the cloudy glands begin to turn amber, then dark as the resin degrades.

If you chose one area and watch it closely you will start to see uncapped trichs.
If you place a sheet of white paper on the soil and the winds don't blow, you will find tiny, black, specks that used to be capitate glands.
They shrivel and drop as tiny specks.
I once collected a pipe full of these mini poppy seeds and gave 'em a try.
Very disappointing, just made me sleepy.

All is not lost though.
The sessile glands are exposed to less UV and last longer.
But, they too will succumb to heat/time/age.

So, my advice?
Harvest anytime after "mostly cloudy" for best potency, and some uppityness.
For a stonier experience wait for some ambering.
But, if you start to see uncapped trichs, it's getting late, chop chop.

Aloha,
Weezard