You can tell if grapes are bad by checking several signs related to their smell, appearance, texture, and taste:

• Smell: Bad grapes often have a sour or vinegar-like odor, indicating fermentation or spoilage

• Appearance: Look for shriveled, wrinkled, or brown grapes. Grapes that have cracked skins with oozing juice or visible mold should be discarded. Brown spots, bruising, or rough patches can also signal spoilage or disease

• Texture: Fresh grapes are firm and should spring back when pressed lightly. Soft, mushy, or overly shriveled grapes are going bad. If grapes feel sticky or have soft spots, they are no longer fresh

• Taste: If grapes taste sour, bitter, or off in any way, they are likely spoiled and should not be eaten

Additionally, if even one grape in a bunch shows mold or significant decay, it's best to discard the entire bunch to avoid contamination

. To keep grapes fresh longer, store them unwashed in a ventilated bag in the refrigerator’s crisper drawer and remove any decaying grapes promptly

. In summary, use your senses of sight, smell, touch, and taste to determine if grapes have gone bad: sour smell, wrinkled or brown skin, softness, and off taste are clear indicators to avoid eating them