We can't tell by the color, smelling, feeling or thumping. If you intend on buying the grapes, the best way I have found is to pinch one off and taste it. If it is good, then you buy that bunch of grapes or take your chances.
By xintexas from San Antonio, TX
This page contains the following solutions.
The easiest and best way to pick the freshest grapes in your local market is to hold a bunch by the stem. Shake gently - if grapes drop off the stem, they have been in storage for too long. If the grapes are firmly attached, are plump and bright, they are fresh . . .
When selecting grapes choose a bunch with a green stem. If the stem is brown, they usually are too ripe.
