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. :-)
I guess I have mainly met older kiwis!
I slightly know Terry Stewart of https://www.classic-computers.org.nz/
from mailing lists, and while I don't follow any Youtube channels as a
rule, I used to enjoy his videos:
https://www.youtube.com/@tezzaNZ
I probably heard more NZ English from Terry than anyone else!
My understanding of shibboleth is that it refers to the primary pronunciation identifying the speaker (which is how it was used biblically).
Those are also interesting but it feels a bit more special to me when it's a secondary speech characteristic because it feels a bit more like a clue in a detective novel -- it almost feels like you can hear inside the author's head and how they vocalise while they write.
In this case, "missle" is a malapropism that is only possible if you don't pronounce the second "i", and so is a secondary "speech" characteristic.
https://en.wikipedia.org/wiki/Shibboleth