i suggest taking a magnifying glass and look at the trichomes.. they go from clear to milky and then to amber.
You want to have them at least milky.. that will give you more of a head-high, while if they go more amber it will be more of a body high..
i pesonally like it when about 5 to 10% are amber, and harvest then, but other people might like less amber or more amber trichomes.