People tap on watermelons to determine if they sound hollow and are therefore ripe; however, this practice does not always meet with success. Instead, look for watermelons that have a smooth surface and a cream-colored underbelly. Despite the best precautions, however, it is difficult to judge the quality of a watermelon without cutting it in half. When cut, indicators of a good watermelon include firm, juicy red flesh and dark brown to black seeds. The presence of white streaks in the flesh or white seeds usually indicates immaturity.