You can't tell when they're done just by looking at the plants and the leaves and the buds. You need to look at the trichomes, the little globules of fluid on stalks that are what make up the sparkly stuff. You want about 10% of them to be amber (overdone), and 10% clear (underdone), with the rest being cloudy or milky white. If you want it to be (very slightly) more of an 'up' high, then harvest a little earlier, and if you want more of a 'couchlock' high then harvest a little later. (That's mainly a strain-dependent thing though, so you won't be able to change the effects much no matter when you harvest. Just a little.)
You'll need a camera with decent macro capabilities, or a jewelers loupe (I use a 30x loupe, got it on ebay, about ten bucks or something), or a microscope of some kind. You can't really assess doneness with the unaided eye; you need some magnification capability.