A jeweler may well be able to tell the difference on appearance alone. So could you, with a bit of practice. This is the cheapest, simplest, least destructive test possible. If there's a colorimetric test, some sort of solution on a swab that changes color on contact with the solid metal, I can't be sure. You might find something in Feigl's spot tests, I'd suspect more likely for tin over silver, on account of reactivity. But beyond that, you can expect tests to be destructive. Unless the pieces are worth paying for looking at under the scanning electron microscope.