I wait for the pistils to curl-up and for most of them to, preferably, turn dark orange. Combine that with the strain's expected harvest time (typically 9-11 weeks for amateur growers) and it's more than a safe bet. I've never seen "amber trichs" on an auto (my experience) and quit looking at the trichs, the pistils and flowers tell you all you need to know - granted that may take a few grows first.
Pistils are white, grow straight up almost, initially, (or gently curl) then they'll almost look dried-up and curly/packed-in when it's done fattening up and growing, and most/many will turn into a darker color. You can see this from a foot away. (commonly known as "hairs" on the bud)
I do get late "bursts" of white pistil growth often, while the rest of the plant looks ready, but if the new growth is going to be minimal, I give 'em a day or two and harvest any ways.
