C822

How could one know that this was not a dream? If it was in the movie, Nolan would hand you a small, resistance-free gyroscope without a second word, so long as you threw that thing on the table and waited patiently for an hour or two. You'd know the answer, of course, if you were patient enough.

If it's a TV show, then it's even simpler. You don't need to have patience and perseverance