1. a word or pronunciation that distinguishes people of one group or class from those of another 2(a) a word or phrase identified with a particular group or cause; a catchword (b) a commonplace saying or idea 3. a custom or practice that betrays one as an outsider

A while ago there was a West Wing episode called “The Shibboleth.” In The Columbia Guide to Standard American English Kenneth G. Wilson notes:

Apostrophes appearing where they ought not to be or missing from where they ought to be are devastating shibboleths in the view of many Standard users, who will penalize the perpetrators mercilessly for them regardless of whether haste, inadvertence, or ignorance caused the outrage against convention. Be warned.