Look at the skin on the grapes. The best grapes will have a healthy shade to the skin. Ripe red grapes should have a dark purple hue, as any signs of green indicate that red grapes are not yet ripe. Ripe black grapes should have a full, rich shade of bluish-black, and green grapes should have a yellowish hue.[3]
In addition to skin tone, healthy grapes should also have a light silver-colored dusting on the skin called bloom.
Bloom is commonly mistaken for dirt, but it's actually the plant's way of protecting the fruit from drying out or rotting.[4]