hey kronic.. just get yourself a 30x jewelers loupe and look at the trichomes with it. you want them to all be atleast cloudy or even half cloudy and half amber colored.. when they are mostly cloudy you will get a more up high, and the more amber that they are you will get a more couc lock type of high.. depending on what type of high that you are after will determine when you want to harvest..
radio shack also sell this lil 100x magnifying glass that you can pick up for like $15 or so, and works great for this as well.. they only problem with this method is that you are only able to see a very small area of the bud at a time as opposed to using the loupe.. you could also use a digital camera for good looks at the trichomes as well if yours has a macro setting. someone i know on here uses hers and gets great snapshots of her trics.
hope this info helps.. i usually just use the breeders recomended flowering period for flushing reasons, and than i just look at the trics for final decision on when to chop them down