need a cheap jeweler's loupe or usb digital mircoscope etc.
can read up on all the sugestions on when to harvest and what it will do to the high, but you should do your own experiments if you grow the same strain or clones again. try a couple days later, if you harvest early etc...
Earliear will avoid breakdown of thc, but may miss out on some thc development etc etc.. you need calculus and many data points to know for sure... :)