Couldn’t you easily detect tungsten by trying to stamp it? Gold takes a stamping easily, tungsten would not yeild. I gotta believe they have some sort of incoming QA to tell the two apart.
As an aside, my wife grew up in Attleboro where there are a lot of jewelry companies. The guy who transported gold for one of the big ones was an elderly man dressed like a janitor who drove it in the trunk of his old jalopy. People (in those days) knew enough not to gossip about things like that and he was never robbed.
If stamping were the only easy test, you could have gold around a core of tungsten and let’s say double your money with a 50/50 bar instead of going for a 10x increase with a 90/10 bar.