There is a test that dearlers can use. Dealers seem to use an acid test, which of course tests the surface. But the surface of these is gold, so it passes. but there are tests that are acoustic or magnetic/electrical which can test for tungsten inside gold. Dealers just need to use them.
I thought Archimedes solved this problem 2500 years ago.
on that Pawn Stars show someone came in with a gold brick and they drilled several holes and did the acid test to the shavings from inside.
It was real.