Best way to know is with 30-60x magnification. You look at trichomes on the calyxes. Google for 1000+1 opinions on exactly when. 25%-75% amber etc etc... Amber is a sign degradation of the psychoactive delta-9-THC. Foggy is what you want, but some portion will likely be amber to avoid large swaths of clear.
currency checkers, usb microscopes etc etc... most phone cameras aren't quite good enough. as little as $5-7 and up from there.