The true way to be deceived is to think oneself more knowing than others.

Confidence in your own sharpness is the opening a deceiver needs. Someone who believes they cannot be fooled stops checking, dismisses the warning from a less clever friend, and mistakes their own quickness for verification. The most elaborate cons run on that vanity rather than on any cleverness of their own. The mark does most of the work themselves.

Where the stakes are high, act as though you were easy to fool. Check the thing, ask the slow question, and tell one plain person what you are about to do before you do it.