During WWII German occupation of the Nederlands, the Dutch used Scheveningen (a seaside town near Den Haag) as a shibboleth to tell Germans from Dutch because Germans

Also if you watch Band of Brothers, the Allied forces use "Flash" / "Thunder" as sign/counter sign (call and response) to verify each other. The reason is because Voiceless dental fractives (th) are rare in German

During WWII German occupation of the Nederlands, the Dutch used Scheveningen (a seaside town near Den Haag) as a shibboleth to tell Germans from Dutch because Germans

Also if you watch Band of Brothers, the Allied forces use "Flash" / "Thunder" as sign/counter sign (call and response) to verify each other. The reason is because Voiceless dental fractives (th) are rare in German 

https://en.wikipedia.org/wiki/Voiceless_dental_fricative

creating and destroying; built flying cars & autonomous vehicles, now building /faust, more here: /july, web: july.rocks

I have Belgian family and they also recount that the Belgians used the words “schelde” and “vriend” to tell native speakers from foreigners in the war.

After 20 years trying out those words at the holidays, I still wouldn’t make it 😆