Watermelon is the perfect summer fruit because it is juicy, sweet, and refreshing, but as with many melons, the fruit's sturdy rind makes it hard to tell whether it's in that Goldilocks zone between underripe and overripe. Here are some visual cues to look for.
Check to see if the watermelon's skin is soggy, and look for patches of discoloration or mold. Watermelon should have firm, taut skin with a light green and dark green-striped pattern, but a yellow or white coloration at the base of the watermelon is perfectly normal and means it is ripe and good to eat.
If you do want to cut into your watermelon, or you already have and you've got leftovers, check to see if the flesh is slimy, off-colored, foul-smelling, or growing something fuzzy. Any of these mean the melon has gone bad, and these traits also apply to pre-cut and pre-packaged watermelons.
To avoid spoilage, store cut watermelon in the fridge covered with plastic wrap or inside an airtight container; whole, uncut watermelons can keep at room temperature for up to 5 days. Cut watermelon can also be frozen, though it will not have a texture just like fresh watermelon after being thawed.