Get some sort of magnifying tool and focus on only trichome heads. They look like a drop of water on top of a spike.
If all of them are at least cloudy/milky, the plant is ready to harvest. But! That doesn't necessarily mean the buds will give you the desired effect that you're looking for.
If you like the couch-lock side of smoking, let around 40%-50% of the cloudy trichome heads turn amber before you harvest.
If you like the cerebral side of smoking, you want to let your trichome ratio to be more on the cloudy side with about 80%-90% being cloudy and the rest amber.
I hope this helps.