The Surefire Way To Pick A Sweet Watermelon Using Just Your Eyes
Can you really pick out the best watermelon at the grocery store using only your eyes? What kind of magic trick is this? Okay, to be perfectly honest, you'll probably need to use your hands, too, but they won't be employed to poke, prod, or otherwise get involved in the judging process. They'll just be needed to roll the melon into position, if necessary, as well as to transport it to your shopping cart when done. As for what your eyes are supposed to do, they should be on the lookout for a yellow patch on the bottom of the melon.
This patch, if you're able to locate one, is known as a ground or field spot. It indicates the exact area where the melon sat in the dirt while growing. A melon with a (partially) yellow belly is likely one that was allowed to ripen fully. No spot, or a spot that's white or greenish, means the melon was probably picked too soon. However, in the words of the wise old sage I'm currently cosplaying, "If that watermelon spot is big and yellow, it'll be sweet, so buy the fellow."
What to do if you still wind up with a dud
If this wisdom comes too late and you've already purchased an unripe melon, this doesn't mean it's unusable. I personally would be happy to take it off your hands, since I've never cared for sweet fruit and deliberately buy produce before it ripens. (So much for that old adage I made up, although admittedly it was for your benefit.) To me, underripe watermelon is no tragedy, and it needn't be for you, either, if you know what to do with it. Underripe melon is at its best in a watermelon salad, since no one expects salad ingredients to be super-sweet. You could stick with the classic melon-mint-feta combo that was so popular a few years ago, but here are a few more recipe ideas if you find those salads too cliched. Unripe melon can also be grilled and then sprinkled with sugar to sweeten it up, while it could also be pickled (and don't forget to pickle the rinds, too).
Of course, it could be that your yellow-bellied melon spent a bit too much time in the sun and, as a result, is overripe. It will likely have a grainy, mealy texture and won't be all that juicy. However, it should still be edible as long as it hasn't started to rot. While you probably won't want to eat the melon as-is, it'll work just fine in sorbet, smoothies, slushies, or other recipes where the flesh goes straight into the blender.