It's not that terribly hard. Tungsten is ferromagnetic, gold is weakly dimagnetic. If you have quite a strong magnet (like a large permanent magnet) and a fine

It's not that terribly hard. Tungsten is ferromagnetic, gold is weakly dimagnetic. If you have quite a strong magnet (like a large permanent magnet) and a fine scale, you can detect the presence of ferromagnetic materials even if present in low quantities. The weakly dimagnetic effect of gold will exert almost no force, and what little it does will weakly repel the magnet. The ferromagnetic tungsten on the other hand will be strongly attracted to the magnet and so can be detected already in trace quantities. You can in fact buy machines for a few thousand bucks that'll perform a barrage of non invasive physical tests on gold to verify that it's actually solid gold. Of course you can't expect banks and gold merchants to go the eye watering expense of actually getting one of those.