Best way to check is with either a microscope or jewelers loop. Check the resin glands (trichomes) and when they are about 75% cloudy 25% amber or 50% 50%, they are ready for harvest
Well, you can find lots of articles online about trichomes color and harvesting. But the basic explanation is when they’re all cloudy, THC is at its highest, when they turn all amber CBD is at its highest. After that they turn brown and CBN is at its highest and will produce quite a few negative effects like nausea. I personally like the 75 25 harvest time. But it’s up to you really and what effects you’re looking for