Picking a sweet watermelon means picking one that has had sufficient time to ripen on the vine and therefore sweeten, without becoming overly ripe and mushy. The old trick of thumping on a watermelon until you find one with a dull thud is not the best way to determine which watermelons are ripe and which are not. Rather, carefully examine the watermelons and make a couple observations. Whether you are in the field harvesting your own watermelons or at the market or grocery store, if a watermelon "passes" all these observations, chances are it is a sweet, ready to eat watermelon.
Press your thumbnail into the rind. It should not easily penetrate. It you can insert your nail easily, move on to the next melon.
Feel the watermelon. It should feel a bit rough, not smooth.
Look at the color of the melon. It should be a dull green color. It should not be too shiny.
Flip over the watermelon and look where the watermelon sits on the soil. It should be a creamy, yellowish color, not green or white. This is most important indicator of a ripe watermelon, according to University of Illinois.
Observe the stem where it is attached to the watermelon. It should be brown and dry. This is true even in the field.
Select a watermelon that is a uniform shape and free from defects, such as bruises and dents, as well. It should also be heavy since watermelons contain mostly water. These will be the healthiest watermelons.
Store uncut watermelons at room temperature for up to two weeks. Only refrigerate when you’re ready to cut and eat them. Cut watermelon only lasts up to two days in the refrigerator if tightly wrapped in plastic wrap.