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
Around here that is called shoplifting, and the store will call the cops and ban you from it. I would not recommend this at all.
I doubt that as most people do it. Stores don't get that picky with one grape. I have been doing it for years in all in different states never went to the pokey yet. Let me know if you have seen a wanted poster of me. Never been stuck with sour grapes.
My husband taught me only to buy grapes that are still firmly on the stem. If there are a lot of loose grapes in the bag, they are old. This has worked for me every time.
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.