@cruet
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
Voiceless dental fricative - Wikipedia
Voiceless dental fricative - Wikipedia