Just a 10X magnifying glass gets you in the ballpark. I have a 20X and 30X magnifyer I use rockhounding that work excellent. I got 'em at the surplus store for $5 each.

All plants are different. Some get heavy resin late, and others the thrichromes are amber when the hairs are still white. As a general rule of thumb I start looking at those buds when half the hairs have receded. I look on the top buds in the light, and down deep in the bud where not much light penetrates. Then I do the same with the lower buds.

On a lot of strains the top buds will harvest a week or ten days before the bottom ones. I try to harvest when 10% of the trichromes are yellowing and most are milky. I hear some people leaving them longer and some will tell you they are ready when the first trichrome yellows.

Any time after those trichromes get chubby and start to turn milky the bud is ready. After that it is a matter individual preference.