Most people go with the reliable “thump” method: lightly tap the melon… it should sound as though your palm is bouncing off the melon. A ripe watermelon can also be identified by slight ridges on the rind. A perfectly smooth watermelon is probably immature.
Some believe another indicator is the presence of bee-stings which cause brown ridges or zipper patterns on the rind. During pollination, bees “smell” the fruit aroma, and bite the melon to bleed the nectar into the rind. Are bee-stung watermelons always sweet? What do you think?