How about when people say they’ve done some unlikely thing? Something they most likely wish they had done?
Then either a: they’re delusional and think they really did it, or b: they don’t think they’re special enough as they are so their need for attention is driving them to try and make themselves special.
To tell which it is, observe how desperate they are in trying to make you believe them. If they’re delusional they don’t need you to validate them, in their eyes it happened. If they need you to validate their memories for them, its because they need somebody to pay attention to them.
I should mention here that I am NOT a licensed psychiatrist, just an armchair scholar with too much time on her hands.
A mythology forum I’m on recently had problems with a guy claiming to be a vampire, saying his mission in life was to unite all the vampires, and that the FBI had gotten wind of it and had tried to poison him. He was . . . most entertaining. Made a nice change from the “Elven Princess Syndrome” that usually runs rampant in such groups.
Sometimes you can ask questions and get people to poke holes in their own ridiculous stories, sometimes all you can do is let them have their delusion. As long as they’re not a threat to anyone, of course.
I don’t know if that answers your question or not, I’ve been kinda ramble-y all night.