The giveaway for me that the story as written is bogus is the "certain detail" (sic) concerning 400 oz tungsten blanks, being plated in a paper thin layer of gold. No way. The fraud would be too easy to discover, even by humble vault employees during normal stacking and counting.
If this story said that 300 oz tungsten blanks were molded inside of 100 oz of gold, it would be more plausible. Then, fraud between willing players might work. (But both sides would have to be aware of the fraud, and have a reason to keep quiet about it.)
But a 400 oz tungsten blank with a few mills of gold plate?
No frikkin way.
I suspect that one could develop a reliable test that involved a computer analysis of the timbre of bars when struck in a controlled way (think xylophone), and the harmonics of a tungsten-loaded bar would be vastly different than anything in the range of normal gold bars.
Then again something like an ultrasound or Xray would probably be simplest.