And then, there’s the webbing. You know, that spider-web looking brown scarring, that a number of melons have? This is surprisingly a good thing! Supposedly it means the bees have gone to town on the flower, thereby producing an over-abundance of pollination, which should theoretically yield some percentage sweetness. More webbing means more pollination, which equals more sweetness? I cannot explain the biology of it, but it seems to work.
Also, and this one always trips people out: watermelons are “boy” or “girl.” Yes, and I know, it sounds like grocery store astrology, but the rounder ones (the “girls”) are said to taste sweeter and the oblong ones (the “boys”) are likely to be more watery. I don’t know if it’s been peer-reviewed or any of that fun stuff, but I’ve had enough old-timers swear by it that I’m not going to dismiss it.
Oh—and this one’s actually useful if you’re looking for a watermelon: look at the stem, or what is left of it. If it’s still green, it probably had to come off too soon. The brown and dried-up tails have had ample time to ripen properly before coming off the vine—those are the ones you want. Not the largest one. Not the shiniest. It just has to be the right size, with the right spot, and that dried little tail.