Despite popular belief, there are a combination of things to look at to tell when a plant is ready for harvest. Looking at the trichomes under magnification is one. Looking at the hairs is another. The hairs tend to turn more auburn/red/brown when they are mature. They also tend to recede back into the buds (for most strains, not all) when they are fully mature.
For trichomes, you need to look at the over all content. Clear THC heads are not ready, immature. Cloudy THC heads are more heady, sativa highs and more amber THC heads are more couch-lock, indica highs. The best is to harvest when you have mostly cloudy and some amber with few or none clear. However, this will be different for every strain. Pure indica strains have a lot of amber at peak maturity, pure sativa strains are mostly cloudy at peak maturity. If you have a hybrid, there should be a variation but usually more cloudy than amber.
Thats a lot of stuff in itself. Then theres the look of the buds by the naked eye. When buds are ready for harvest, they a golden sheen to them, like they are perfect or some shit. That is what I look for right before harvest. Good luck!