When choosing a watermelon from the grocery store or a farmer's market, there are two signs I look for to determine the best one to pick. If the watermelon is completely green all over, it isn't ripe. The best watermelons are ripened on the vine and they develop a yellowish look where they sit on the ground, so look for one that has a yellowish area on the bottom. The other way is to tap it. A ripe watermelon will have a hollow sound to it when tapped.