A word used as a test of belonging

I remember a podcast a while ago where Grant talked about a word with Biblical origins that describes a linguistic test to determine if a person belongs to a certain group/culture/ethnic group. I believe he also gave an example of this type of test used by soldiers in WWII, specifically to see if a person pronounced the letter W as a V sound.

Can anyone help me remember what that word is? I'm trying to describe this to a friend of mine, and I'm drawing a blank on the actual word!

I'd love to hear that podcast again, too. It was a good episode.

Sarah