#1 – Check the Field Spot on the Underside of the Watermelon
The underside of the watermelon where it touched the ground should be buttery yellow to dark yellow in color. This is called the field patch or field spot. If the field spot as pasty white as a bald guy's head in the middle of a Wisconsin winter, it probably hasn't reach peak ripeness
#2 – Check the Sound to See if a Watermelon is Ripe
This is a classic way to pick a good watermelon, and the internet is filled with descriptions of how a ripe watermelon should sound. Most say “flat” or “dull”. I think that's a poor description. For my part, “flat and dull” is the sound you get when you rap on something like a giant zucchini – or an underripe watermelon.
A ripe watermelon should have a nice, deep hollow sound, more like a drum or knocking on a door.
How to Store your watermelon
Melons like it warm! Storing watermelons at room temperature also makes them more nutritious.