get a 30x scope or a jewlers loop (like $10), there is really no other way to tell perfectly. If i am going to be smoking the product i like to wait until it has ~40% AMBER tri's, this will yield a heavy more narcotic feeling high where if u harvest at the first sight of amber tri's when most are milky u will get a much more sativa like high.
I cant say much for THC content because i havn't tested many of my runs, but i did notice that the buds dont seem to put on more weight after amber trichromes begin appearing. Early picked weed is some peoples favorite, so its all preference, and i don't think THC content is responsible fore the difference in high, my guess is THC doesn't change that much.
Also, it is probabl that your plants wont all finish at the exact same time, especially if u have diff strains, so check them all dont just start hacking cuz ones ready.