You may not be able to tell if the watermelon that you bought at the supermarket is ripe. Suzy Karadsheh, bestselling author of the New York Times, shares some simple tips to make this watermelon harvest the best yet. If the watermelon sounds hollow, then it is probably ripe. If the watermelon has an oval or oblong shape, it is probably not yet ripe. The third thing to look for is a dull rind. If the rind is shiny, it means that the watermelon has been picked too soon. Feel the weight of the watermelon. A watermelon that is ripe will be heavier than its size. Hold the watermelon to your ear, and shake it. You should be able to hear the seeds inside rattling. You should hear seeds rattling around inside. If not, you can put it back.