The word you want is shibboleth. From Wikipedia:
The term originates from the Hebrew word shibbólet (שִׁבֹּלֶת), which literally means the part of a plant containing grains, such as an ear of corn or a stalk of grain or, in different contexts, "stream, torrent". The modern use derives from an account in the Hebrew Bible, in which pronunciation of this word was used to distinguish Ephraimites, whose dialect lacked a /ʃ/ phoneme (as in shoe), from Gileadites, whose dialect did include such a phoneme.
Recorded in the Book of Judges, chapter 12, after the inhabitants of Gilead inflicted a military defeat upon the tribe of Ephraim (around 1370–1070 BCE), the surviving Ephraimites tried to cross the Jordan River back into their home territory and the Gileadites secured the river's fords to stop them. In order to identify and kill these refugees, the Gileadites told each refugee to say the word shibboleth. The Ephraimite dialect did not contain the "sh" sound and so those who pronounced the word as sibboleth were identified as Ephraimites and killed.