How did they get their hands on silver and gold in the first place? Although quite a few people may have silver or gold jewelry, exceedingly few have coins or bars stashed away... and they would face an uphill battle trying to buy anything with that, as most of their trading partners wouldn't have the faintest idea how much a gold coin is worth or how to tell if it's real.

Gold and silver may be OK as a portion of a "rainy day" portfolio, but mostly to protect against hyperinflation or deposit confiscation, with the expectation that you would still exchange it for currency down the line. But it's not a particularly credible foundation for everyday trade, not in the modern day.

Anyway, there's plenty of ways to tell, and the most robust include ultrasonic thickness gauges (sound waves propagate at different speeds in different metals), conductivity testing (e.g., Sigma Metalytics), and x-ray fluorescence (expensive). More traditional methods include various destructive or semi-destructive tests, including acid, hardness measurements, etc. Sort of depends on how much you want to spend. X-ray fluorescence equipment is really costly.