> were literally worth their weight in gold (or silver, copper, bronze, whatever), so it was probably easier to pay with foreign currency than we might assume

Are you sure you know what the coin paid you is made of? A merchant of the time wasn’t. Those who care not to be scammed have never found it simple.

https://en.wikipedia.org/wiki/Debasement

Experienced traders can make a quick estimate of the purity by rubbing it against a touchstone, which has been used since ancient times. And by treating the rubbings with mineral acids you can make even more accurate determinations, although I'm not sure if this was done in the 1620s.

https://en.wikipedia.org/wiki/Touchstone_(assaying_tool)

You are discussing absolute certainty, but in practice a box full of Spanish dubloons was very likely to be a treasure trove, and people generally trusted coinage, even if they had doubts. A filed silver penny still often bought a penny's worth of goods.

In practice though, you only have to be as confident as the guy who will eventually sell you something for it.