Once the plant has atleast 50% change in the color of the pistil hairs from white to an orangish-red as well as looking very closely at the trichromes. This can be done with a jeweler loupe or I use a small pocket magnifier I got at radioshack for 12$ it goes from like 60x to 100x.
Your looking at the small resin heads on the buds and smaller leaves. You can think of resin production much like a volcano when it is flowing freely it will be clear and as it slows down it will become milky and finally as it cools or in our case stops production then it turns an amber color. During the flowering cycle the plant can slow down and increase production of resin so milky color alone is not enough to determine ripeness. It can be difficult when to judge when to start flushing to reach an optimal flush time before the trichromes turn to far amber. You can basicly start flushing at the first signs of amber heads and harvest at a 50/50 milky/amber ratio. This will produce a head high as well as a body high, go more clear and milky and it will be more heady, go more amber and it will become more of a body high with couch lock. Ofcourse, this all has to due with the maturaty of the THC and other chemicals the plant produces and how your body breaks them down.
Hope this answers some of your questions and if your looking for some more info on reading trichromes then I can point you to some more detailed threads.