A shibboleth is the distinguishing feature of a particular group of people. The term comes from the Old Testament:
And the Gileadites captured the fords of the Jordan against the Ephraimites. And when any of the fugitives of Ephraim said, “Let me go over,” the men of Gilead said to him, “Are you an Ephraimite?” When he said, “No,” they said to him, “Then say Shibboleth,” and he said, “Sibboleth,” for he could not pronounce it right. Then they seized him and slaughtered him at the fords of the Jordan. At that time 42,000 of the Ephraimites fell. Judges 12: 5-6
Brutal, man. So the word 'shibboleth,' which refers to the stalk of a plant in the dialect of the Gileadites, contained the 'sh' sound, but the Hebrew dialect of the Ephraimites did not have that sound. They would give away their accent by pronouncing the word incorrectly and thus would reveal themselves to be Ephraimites.
When you speak a language natively you have an incredible ability to pick up on tiny variations in speech that distinguish nonnative speakers or speakers of separate dialects. But it is extremely difficult to pick up on these differences in a language that you are learning. For example, few of my students and teachers are able to distinguish American and British English unless they hear specific words that they know are pronounced differently, like 'schedule.' In the same way, I am told that it is very easy for Nepali speakers to recognize native speakers of Tamang and Newari by their accents, but when I am speaking to Nepalis in Chapagaon I am completely unable to pick up on these variations. I am interested in the multilingual classroom dynamic, but because students tend to speak Nepali in the classroom (and, if I'm having a good day, English), it is difficult to tell if an individual is a native speaker of Nepali, Tamang, or Newari. So I asked the teachers to give me shibboleths for Tamang and Newari speakers (with much better intentions than the Gileadites had for the Ephraimites).
For Newari, I was told, speakers often mix up dental and retroflex 't.' These are the two sounds designated by the separate Devanagari characters त and ट, and distinguish a 't' with the tongue placed on the teeth with the 't' in which the tongue is curled back. Both are slightly different than the English sound.
Tamang speakers, I was told, say hunchhaa instead of hunchha (hunchha is sort of like the English 'okay'). In Romanized Nepali, people usually spell the vowel sound अ as 'a' and आ as 'aa.' The difference is crucial in distinguishing between the Nepali verb marnu 'to die' and the verb maarnu 'to kill.' In fact, I heard a rumor of a court case in Nepal in which a nonnative speaker of Nepali on the stand caused someone to go to jail because he accidentally referred to the victim as the maareko man (the killed man) instead of the mareko man (the dead man). Don't know if that's true, but Tamang apparently also lacks the distinction.
Also I was told that Tamang speakers will accidentally say mero baa garchha instead of mero baa garnuhunchha for 'my father will do (this).' In other words, they will forget to use the respectful verb form when referring to their elders. Like Spanish and German, Nepali has both informal and formal you (actually several levels of formality, as I mentioned here). But unlike Spanish and German, Nepali also has informal and formal he/she. So you must use the respectful form not only when talking to your father, but also when talking about your father. Tamang apparently lacks the distinction, so Tamang speakers will forget to use the respectful form when speaking Nepali.
Interestingly, all three of these distinctions are problematic for English speakers too. I often use the wrong vowel, mix up the two t sounds, or forget to use the respectful form when talking about my parents because English also lacks these sounds and grammatical forms. But nobody ever mistakes me for a native Newari or Tamang speaker because there are many other aspects of my accent that give me away as a foreigner (also people tend to pick up on that by looking at my face). I'm just covered in shibboleths.