Grapes (Vitis spp.), which thrive in U.S. Department of Agriculture plant hardiness zones 5 through 9, do not continue ripening once plucked from the vine. If you were an expert grape grower, you could use a special tool to measure sugar content, acidity and pH to determine when your grapes are ready to pick. As a home grower, however, you can just use your natural senses to determine when the grapes ripen.
Color alone does not indicate the ripeness of grapes, although it is an indicator. The correct color depends on the type of grape; some ripen to a deep purple, almost black color, some to a dark red and others to a golden green. All varieties will form a noticeable whitish coating when ripe. If you have seeded grapes, you can also check for readiness by examining the seeds: if the seeds are brown, then the fruit is ripe; if they remain green, the fruit needs more time on the vine.
You can further test the ripeness of grapes by gently squeezing one at a time between your thumb and forefinger. Grapes that still need time to ripen will feel firm and won't budge under light pressure. Ripe grapes will give just a little with a squeeze due to slight dehydration and softening of the pulp. If the skin looks shriveled and the grape crushes easily, it has moved past ripe.
When you eat a ripe grape, it should burst with juice, and taste of equal parts sweet and tart. If you pick a grape, eat it and detect any bitterness or raw vegetable flavor, the fruit needs more time on the vine. If the grape looks and feels ripe, this is the final test. It's also one of the quickest and most direct methods for testing grapes -- and the most dependable across all varieties.
If you don't harvest the grapes as they ripen, birds may get to them. As long as they do not decimate your grapes before you and your family can enjoy them, their presence can be a good indicator of readiness for harvest. If the birds become a problem, however, you can throw a net over the fruits to protect them.