(1) it depends considerably on what variety of grape you are growing what it looks like when it is ripe. The date of ripeness can vary considerably from one year to the next, as a glance at the harvest dates of any major French wine region will indicate.
(2) even major wine-grape growers will pick and test samples of grapes in order to decide when to harvest. Whilst the professionals will have a battery of chemical tests, for the amateur a simple test is to taste some.