I'm wondering the same thing now with my plants. I'm a first timer and am basically eyeballing pictures of buds that are ready for harvest vs buds that aren't. The ones that aren't ready still have healthy white pistils, or just the tips have started turning reddish and curling back. Seems like after the pistils have completely whithered back, and are reddish is the first indication that it's getting close. Then I guess you should look through a scope (30x mag at least) and see if the trichs are cloudy or amber. If they're clear, you probably wont get the high you want.