A shibboleth is a little more specific: it's the pronunciation of one
  specific sound in a different way that marks out a person as from one
  specific group. 

  So, some of the go-tos... New Zealand English tends to turn an /i/ in
  even stressed positions into schwa (the sound on the end of "little",
  or "umbrella", or "gopher", and one of the key sounds to master for a
  non-native learner who wants to improve their English accent). When a
  Kiwi says "fish and chips" it sounds like "fush and chups", "swimming
  pool" becomes "swumming pool", and this is _the_ way to distinguish a
  Kiwi from an Australian, because to the rest of the Anglophone world,
  NZ English sounds very like Aus English.

  Canadians tend to pronounce the "ow" sound in words like "about" in a
  way that to my British ear sounds Scottish: it becomes "aboot". So if
  I hear someone speaking with an American sounding accent but they say
  "I found oot aboot something," I ask them if they're Canadian. I tend
  to find most Canadians and New Zealanders are delighted if someone is
  able to tell, so ask very early on and you can make new friends. :-)