How can I tell which grapes in the store are ripe and wholesome?

Grapes are harvested only when fully ripe. Fully-ripened green grapes should have a yellow cast or straw color with a touch of amber. They should not have an opaque grassy-green color. Red grapes should be a deep crimson rather than milky or pale red. Blue grapes should be dark, almost black rather than pale or tinged with green.
 
Grapes should be plump. Avoid bunches bearing underdeveloped, very green fruit. You can always judge the freshness of grapes by the stem. The greener the stem, the fresher the grapes. Grapes should always be firmly attached to their stems.
 
Source(s):
Whole Foods "Guide to Fruits"
Tonytantillo.com "Fruits: Grapes"