The best indicator of when to harvest is trichome ripeness. The trichomes turn from clear to milky white and eventually to amber as the buds ripen and the cannabinoid profile changes.
The harvest window starts when most or all of the trichomes are milky and ends when most or all of the trichomes are amber. Some sativa strains will never develop amber trichomes however. At the beginning of the window the effect of the cannabis will be more uplifting and euphoric, and at the end it will be more sedative and analgesic.
You can look at the trichomes with a loupe or magnifier (at least 40x) or a microscope, or if you have a high megapixel camera (about 20 plus) you can take a photo under strong neutral lighting and zoom right in.