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