We must accept one premise that comes with being a ghost.
That premise is: you must be dead.
If you are already dead, you cannot die a second time.
So there is one way to prove ultimately that you are a ghost.
You must attempt to kill yourself by means that there is 0% chance of survival.
If you die, you become a ghost (maybe), and you now have proof.
If you do not die, you can be certain that you are a ghost.