The first thing to look for is a rotten spot on its skin when choosing a plum. Since plums have a short shelf life, they are often picked before they are fully ripened. The skin will become very soft, develop dark spots, and ooze. The flesh will also begin to turn brown or have an unpleasant odor. These are all signs of spoiled fruit.
A ripe plum can be kept fresh for three to five days in the fridge. It can keep for a week or more before going bad.