One of the most important decisions for a winemaker is when to harvest. Grapes should be harvested in their full maturity; because better the grapes better the wine.
But how do you know the exact time?
Well, you can start by using your senses – see, touch, smell, and taste grapes.
After that take a representative sample from the vineyard and make measurements of sugar content, ph, and acidity level. You will probably have to repeat measurements several times, before the majority of grapes will be ready for harvest.
See the detail explanation on when to harvest your grapes in the video below, given by Tom van der Linden from the University of Minnesota Extension.