What you need is a 60-100X microscope. Trichomes colouring is not visible to the naked eye.
What you can do is a less scientific approach and judge ripeness based on the pistles; in which case it is less accurate.
Judging on pistles alone would indicate that this could be harvested but might not be at peak potential THC.
Note that jewelers loups are most commonly 30-40x, enough to inspect for mites and other infestations.
Trichomes are most easily inspected under a 60-100x mini microscope. Available at your local grow store.
Percentage of ripness can be played with. I suggest aiming for 20% amber as an average. 10%- will be a speedier, shorter lasting high, and a 30%+ will be the heavier chouchlock feeling.