This leads me to an interesting notion (I strongly suggest you don't take this too seriously
)
Someone who is way psychotic and hear voices and see things, firmly believes all this is real. Can you make an absolute guaranty to yourself that you are
not in fact say, a person of the opposite sex, 67 years old, living in the 16th century, hopping mad and locked up in a high security asylum somewhere, at this very moment?
Ok, on a serious note: I believe that "if you are right and everyone else is wrong, then you are mad" holds a great deal of truth. I think this will mostly only apply to the real nutcases, who find themselves opposing everything everyone else hold as true. Just because you disagree with everyone on one issue only, doesn't mean you're insane, but as the number of issues grow, the reasons to worry should also increase.
Let's see if this works:
"If you are always right and everyone is always wrong, then you are insane".
Leads to: "If you are not always right and everyone is not always wrong, then you are sane"
Yup, it seems fair to me.
Also: If you worry too much and question too much your own sanity, that doesn't necessarily mean you're going insane, but it means you are in some sort of trouble and ought to have a look at it before you walk up to the Dottyland Border and Customs Office.