Picking the perfect watermelon can seem like a shot in the dark. To pick the perfect watermelon every time, there are a few key things to look for. Watermelon should have a hard rind. The watermelon is ripe when it has a yellow belly. When you find one like this, go ahead and buy it! Do not pick a watermelon that has a white belly or no color on the belly. The watermelon is not ripe and will leave you disappointed by the lack of sweet watermelon flavor.