Bunches of red and green grapes at a market
The Correct Etiquette For Sampling Grapes At The Supermarket
Some grocery stores will encourage people to test grapes before buying, and as long as you follow proper etiquette, it is acceptable to try out your potential bag of grapes.
Ask an employee if you can sample a grape to see if it's ripe enough, and they'll probably oblige. They may even provide a glove to make your sampling experience more sanitary.
Limit yourself to one or two grapes, and don't place a half-eaten grape back in the bag. It's also worth noting that this taste-test etiquette does not extend to larger fruits.
If trying out an unwashed grape doesn't appeal to you, you'll want to look for grapes that have a bright color and are firm and plump. They should come off the vine easily.