The hairs is a solid method but everywhere i read the best way to do is follows:

Get a hand held magnifying glass. Handscopes is what they're called at radio shack (10-15$)
Zoom in on the Resin Glands and check their color. If they're still clear then its to early. When they're a dark amber its a very body heavy high. When they're a cloudy milky color its much more of a head high. Given that one generally harvests based on the preferred high
Mic-Mechanic Reviewed by Mic-Mechanic on . Outdoor/Indoor Grow - When should I harvest..? Hi, this is my first grow and could do with a little advice. I was given a blue cheese clone by a friend a few months back for medicinal purposes, which i stuck it in a big pot with some pearlite and some decent fertilizer, and left it in the garden. The uk had a pretty wet summer this year, but a few weeks of sunshine late in august seemed to do her well and she shot up, and mid september i decided to bring her inside to avoid the frosts. I've attached and image taken on the 23rd Sept, when Rating: 5