A good overall resource is: http://www.crannogales.com/HopsManual.pdf
...go to Harvest Time
(pg 29) concerning when to harvest.
I like to look at the hops to test.
1. Take a hop, squeeze it and it should spring back into shape immediately.
2. Then I open the hop from the top down and I take a look at the color and amount of lupulin. I'm usually looking for the yellow lupulin to start to turn towards a yellow-orange color. Different hops hold different amounts ...eg: Hallertau less then Cascades. It is tricky sometimes ..if you wait too long, they can go "cheesy" on you and you don't want that.
3. Then I usually rub the hop between my hands really fast to warm up the lupulin and smell it. The smell should be quite noticeable, appropriate to the hop type, and my hands should be nice and sticky form the lupulin.