If you use traditional sterling for welding, you’d have to be
positive that all solder was removed as sterling wire dislikes
melding with solder. If you didn’t want to remove the solder and the
piece is plated, I would recommend the Argentium solder wire since it
melds better to other silver alloys.
If the piece isn’t marked “Sterling” or.925", that’s an indication
it’s plated. It’s to the manufacturer’s advantage to advertise the
alloy. If that piece came to my shop, I’d scrape along the ring where
it was attached to the other side. I would then put a piece of
glossy white paper or plastic next to the area to determine if it’s
plated over white metal (it will scrape like butter), copper, or
brass. If I still couldn’t identify the base metal, I would perform
an acid test.