I expect that this post is just a rehash of Fernando's above.
From a bit of research I have found that nondescript foriegn coins in that age that were made from gold or silver were valued by weight. I expect innkeepers and tavern owners kept a small balance scale and weights. If one knew (for example) the value of a Thaler then the equivalent weight in silver could be adjudged as the same.
Purity and counterfeit might be a problem; I always think of the image of a merchant biting a coin to asses its purity.
