You know when you're awake don't you? Do you always know when you're dreaming? Or remembering something that never happened?

There is such a thing as "false memories".  Ideas implanted via suggestion? Of course we know about hypnosis and what that can do. THE MANCHURIAN CANDIDATE exemplified that.

So how do you know what you "remember" happened? Maybe it was something in a movie you saw as a child and it wasn't YOUR  at all but one that was purely invented as entertainment.