Put the ring on a sheet of aluminum foil. Spit on it. Close the foil around the ring and spit. Rub it slightly (kind of push the moisture aluminum and ring together. Let it sit for about 15-30 seconds.
Open the foil.
If it stinks like bad eggs (sulfur), then you've got silver.
I know... sounds VERY weird... but it works! Cheapest Silver test I know of. Junk rings won't stink.