You can tell that the raisins have gone bad by following the points below.  As usual, start by looking for any typical signs of deterioration, such as mold or any other organic growth. A bad smell (instead of the usual sweet and sour aroma) or significant changes in color means that the raisins are spoiled and should be discarded as well. If everything about raisins seems perfectly fine, eat one or two and decide what to do with them based on the taste.