The bracts (calyxes) are the things with the pistils sticking out of them. Those stay even after the closest trim and have the most trichomes, so those are the ones to check.
Also, the sugar leaves will amber first.
Speaking of amber, since you're growing outside, don't pay as much attention to the trichomes as indoor growers. Note how often you need to water and see when it really starts to slow down the drinking. That and a change in smell are better indicators outside of when harvest time approacheth!