If you purchase a quarter of a watermelon already cut, you can see whether it's ripe. However, if you start with a whole watermelon, there are three things that you can do to ensure that you've purchased a ripe and juicy watermelon. The field spot is the underside of the watermelon, where it rested on the ground as it grew. A ripe watermelon will have a creamy yellow field spot, while an unripe watermelon will have a white field spot. When you thump the watermelon with your knuckles, it should sound hollow. The watermelon may be overripe or not ripe enough if it sounds dull. Finally, the rind of a ripe watermelon should be relatively smooth and should have a dull sheen. If the watermelon is shiny, it might not be ripe yet.