How do you tell if a grape is ripe?
If you answered, “by the taste”, you are right!
But, there are also other measures of ripeness. For starters, there are the characteristics of the seeds. When grapes are unripe, the seeds are green and as they ripen they become tan and finally turn brown. Also, before ripening, the seeds are chewy and the pulp clings to the seeds but when ripe, the seeds become crunchy and the pulp no longer clings but separates easily.
If you are a winemaker, there are also other factors – chemistry stuff like pH and Brix.