When someone says they know the answer, you can take it to the bank they don't. Saying you know the answer is analogous to saying, "I'm not human." No one can know the answer. Which isn't to say that people can't say, "This is the way it seems to me." Or "I think this is what that is." But "I know." Run from those people like the plague. Why?
Because they are going to insist that you see things their way. Why wouldn't they? They're sure they know the answer. They've got it and they're sharing it with you.
Would you reason with them? Maybe make them see a different side of the story? Don't waste your breath. They know the answer already. All they want from you is your agreement that they're right.
And sometimes such "knowers" seem enviable in this world of doubt and uncertainty. But don't fall for that trap. People who "know," no matter how seemingly together, are deluded. They are losers.