That's why it makes senss to only buy gold cast in a thin shape (like a coin). It's much harder to cast in any meaningfully large tungsten core and it significantly raises the likelihood of that fake core being discovered as one can verify the authenticity of the gold piece by scraping away a millimeter or two of gold using basic hand tools.

Considering how expensive gold is per ounce, it's not like any of us will be buying it by the bar.