wine guys: how do you know grape quality before you pick?

How do you know the grapes you are getting ready to harvest are correctly ripe and ready to pick? do you sample some grapes and do a lab analysis on them for acidity, sugar content? Do you just look at them and “say…yep, they’re ready”
I was just wondering in the profession wine making world, how they know when things are ready to pick

Sugar content is easily measured in the vineyard with a device like this.

My BIL is a winemaker and they do sample during the growing season but he can also tell pretty well if the harvest is going to be of good quality just by observing the vines. He’s there every day so he knows what the vines have been exposed to. There’s a race near the end to get maximum production and then harvest before the rains fall.

It’s a delicate balance of sugar content, acid levels, pH, tannin ripeness, and flavor. These don’t all reach their optimum value at the same time. Usually you use a combination of field measurements (like in zombywoof’s link), detailed lab measurements, and your own senses. See here for a lot more details.