You need a good magnifying glass to see the resin glands. They will swell like little mushrooms under magnifacation and the optimum time to harvest is when they are turning a milky coulour.
The colour of the pisils is a good guide when they are 60/75% brown.
If you leave it too long the plant will start to produce another drug called "CBN" (abriviated) .
This will give you a very heavy stone and tend to knock you out rather than give you a more social stone .
Some people like this , I personaly don,t so I try to pick on the early side rather that the late.