Shibboleth

A shibboleth is a word or phrase that can be used to determine a speaker's social or regional origins.

I remember my father once telling me that Germans asked people to pronounce the word Kirche, which apparently those who are not native Germans find difficult to prounounce.

Although I've known the concept for a long time, I never knew the term that described the concept - and learnt it while watching an episode of The West Wing

Wikipedia