When selecting a watermelon, pick the melon up and ensure that it feels heavy for its size, even if it is a small melon. Look for a splotch on the watermelon; this splotch forms from the melon lying on the ground and should be yellow if the watermelon is ripe.
This yellow spot is called a "field spot" and indicates that the melon was allowed time to lay on the vine and ripen in the sun; white splotches can be an indicator that the melon was harvested prematurely. The underbelly of the watermelon also holds clues to its ripeness; if the underbelly sounds deep and hollow when you tap it, then it is ready to eat. If it sounds dull when tapped, it is too ripe or not ripe enough.
Other things to look for when selecting a melon include its skin color. Choose a melon that is dull in appearance; if the outside is overly shiny, it can be an indication that the melon is not yet ripe. Inspect the melon, looking for cracks, soft spots, bruising and moldy patches. Generally, the best watermelons are those that feel heavy, have the splotch indicated above and have a hollow sound when thumped.
The chosen melon should not be narrower on one of its ends than the other. Misshapen melons may have grown in spurts and may lack the best flavor.