Commonalities Of Accounts Of A Universal Flood?

When a myth or story of another culture deviates from the Bible, it's godless, sinning trash. But when it agrees, then that OBVIOUSLY means the myth is true.If the universiality of a myth proves its truth, then couldn't a non-universal myth be deemed untrue?

Then how do you know the Bible is the one true source direct from God? Maybe it's just another "demon-inspired" myth.Or maybe Satan and a whole gang of demons decided to convince everyone on the planet that there was a world wide flood. You know, like celestial Punk'd.

Say there was a worldwide flood. Only a few people survived by jumping into a large boat. When the flood receded they told the story to their children, and they told the story to their children, and so on, until the story is spread across nearly the entire world.Now say someone made up a story about a giant flood, told it to their children, and they told it to their children. and so on, until the story is spread across nearly the entire world.How would we tell the difference?