do you simply use a Micro scope
On almost every plant I do a multi stage harvest because very few of them are ripe throughout the entire plant. This could mean two harvests or as many as 4 or 5 harvests depending on the plant.
Its why its imperative to keep as many healthy green leaves as you can for the entire grow.
I have let the bottoms of plants go for 2-5 weeks after the start of harvest. After you cut the top of the plant you raise the lower part closer to the light and let it go for a while. You will increase the overall yield and those buds will be as potent as the top buds.
I go by the look of the plant to my eye. Then i look with a scope.
Each plant is different so you will eventually find the optimum times for harvest. You will find that there is a point where you get diminishing returns and 3 weeks might be better then 5 weeks.