via What About a Watermelon
2. LIFT – The ripest watermelons have the most water. And since watermelons are 92 percent water, your watermelon should be relatively heavy for its size.
3. TURN – Turn your watermelon over and check out its bottom, which should have a creamy yellow spot (also called “the ground spot”). This is where the watermelon sat on the ground while it soaked up the sun at the farm. If this spot is white or greenish, your watermelon may have been picked too soon and might not be as ripe as it should be.