I have heard from many that the ripeness of watermelon is determined by the dry tail. But if the watermelon is broken, then it is natural that the tail withers.
The ripeness of watermelon is determined by two features.
At the base, where the beginning of the tail of watermelon, there is a small leaf. If it withered, then watermelon ripe. I pointed at him with a red arrow.
The skin of the watermelon should shine.