Shibboleth

A word or phrase that can be used to identify whether someone belongs to a given group. The word comes from the Bible's Book of Judges, where one tribe could not pronounce word Shibboleth the way their rival tribe did, and thus it was used to expose enemies, Kind of like Danes asking foreigners to try to pronounce the phrase: "Rødgrød med fløde," although that's usually more for laughs.