The famous example from the movie Inception is spinning a top; if it never stops you are probably asleep (or maybe reality broke, oops!). But most people don't

The famous example from the movie Inception is spinning a top; if it never stops you are probably asleep (or maybe reality broke, oops!). But most people don't carry around a top to do this, so the other replies give some more practical examples that LD practitioners have come up with over the years.