The best way I’ve found, which works like 90% of the time, is to knock on it while holding it. It should sound somewhat hollow, but not empty. Too hollow means it’s not juicy and not hollow enough means it’s under ripe. Tells you nothing about flavor, but it at least allows you to pick one that is decently juicy and ripe. It’s all relative but if you knock on a few you’ll quickly find one that sounds in the middle. Pick that one and you’ll have at least a decent watermelon.
The best way I’ve found, which works like 90% of the time, is to knock on it while holding it. It should sound somewhat hollow, but not empty. Too hollow means it’s not juicy and not hollow enough means it’s under ripe. Tells you nothing about flavor, but it at least allows you to pick one that is decently juicy and ripe. It’s all relative but if you knock on a few you’ll quickly find one that sounds in the middle. Pick that one and you’ll have at least a decent watermelon.