I assume there is no casting seam?
With it being that darkly toned, not sure it would pass the
tissue test.
One thing you could try is a heat conductivity test. Never tried it personally, but on paper a silver coin should get hot or cold much more quickly than say a cupronickel coin. So in theory, if you were to hold this coin against an ice cube, you would feel it get cold perhaps a second faster than a nickel. Problem is, the denarius in this time frame (IIRC) was about 55-51% silver, so you would have to find a 3 gram, .500 coin to compare.
At any rate, here is the wildwinds page with seven genuine examples of your coin:
http://www.wildwinds.com/coins/sear5/s6819.t.html