When you figure out someone has been lying to you by either common sense, street smarts,or investigative skills, much like famous detective Nancy Drew. Tends to lead to total ownage by said investigator, and utter humiliation by the perpetrator.
Liz: Omgz! I knew she was lying!
Liz: I found his real myspace! The other is a fake!
Sarah: You dont say!
Liz: I do say! I've put all the clues together. This seals the deal, nukka.
Sarah: Well, you just Nancy Drew'd her ass.