Look at the grapes. Ripe white grapes should have a translucent appearance and a slight yellow tint. Ripe red grapes should be dark red in color, with very little color variation in the bunch. Look at the seeds inside your grapes. Ripe grapes should have round, brown seeds; if the seeds are green or very light brown, the grapes are not ripe enough to eat. Look at the stalks on your grapes; they should look fresh and new. Dead stalks likely indicate overripe grapes.
Taste several of the grapes (tasting one or two will not be enough to tell if an entire bunch is ripe). The grapes should be sweet, with none of the sharpness associated with high levels of acidity.
Shake your bunch of grapes. If more than one or two fall from the stalk, the grapes are likely to be overripe.