Scenario: You wake up and you're not where you usually are when you wake up. Instead you're in a psychiatric institution as a patient under a different name other than your own. You still have all your memories and knowledge of your life up to this point, and in your mind you're still sane. However, the belief in your own mind won't be enough to save you from this weird scenario, you have to convince one of the doctors of your sanity.
What can you think of to undoubtedly prove your sanity?
Note that rambling on about your "real" life won't be enough. You may well have been in work yesterday, but screaming "Call my boss! here's his number..." won't help, the point is to prove your sanity to the doctor, because until you do, he won't willingly help someone believes to be insane.