A 30x jeweler's loupe (ebay) also works okay, and any modern camera with a decent macro mode should work okay too. Here's a pic of some trichomes, they're the tiny globules on little stalks that coat the leaves and buds, and they change from clear to cloudy to amber over time. You want about 10 percent or so of them to be amber when you harvest. You can ALSO see some red hairs in the pic, but those aren't relevant to when the plant is ready to be harvested. (Pic taken with a Canon A620 in macro mode.)