1. Pick a dull looking watermelon. A shiny appearance indicates an underripe melon. This applies to honeydew melons too.
2. Find that field spot. This is a creamy spot on the melon, and it's where the watermelon was resting on the ground. The field spot should be a yellowish creamy color Like the pic)
3. The darker the color of the creamy spot, the longer it was on the vine sweetening up. If it is white (or not even there), put it back, because this indicates an underripe melon.
4. Knocking.. A dull thud indicates an underripe melon. You will get a dull thud if the flesh is soft, which you don't want. Your knuckles should bounce off the melon, and the surface should be pretty hard.
5. Pick it up. Is it heavy for its size? Note: this applies to pretty much ALL produce. When you pick up onions for example, pick the heaviest one for its size. That means there is lots of water in there.
While you've got the watermelon in your arms, make sure it's uniformly shaped. Irregular bumps indicate it may have gotten inconsistent amounts of sun or water.