When grapes are going bad they'll usually start to smell a little like vinegar.
Also when grapes are going bad you might notice they begin shrinking a bit and becoming wrinkled instead of smooth and may also begin growing mold on them.
To keep grapes from spoiling too soon you should always keep them refrigerated when not eating them because when you leave grapes out at room temperature the grapes can begin to go bad quickly and start to grow mold and bacteria on them.
When grapes are stored in the refrigerator the grapes last up to 14 days but if you leave the grapes out at room temperature then they usually only last a few days at most.
So always keep the grapes refrigerated to keep them from going bad and also avoid washing the grapes until you're ready to eat them as that can make them go bad sooner.