The best time to harvest a marijuana plant is when about 50% of the trichomes have turned from a milky translucent color to amber. the what colors they should be at harvest time. Not all the trichomes will mature at the exact same time, so harvest when most of the plants are ready. Or harvest each bud separately.
It is nearly impossible to see the trichomes with the human eye, so some type of magnification is needed. An inexpensive way to effectively view them is to use a pocket microscope rated at somewhere between 20x to 70x.
There is another type of trichome that can be found on marijuana plants, they are called cystolith trichomes. This type of trichome is mainly found on the underside of leaves. They look like small hairs with no resin gland at the top. Cystolith trichomes are not a source of THC.