I have a ring that says 18kt but then there is in uppercase H G F O or H C F O or something like that can u tell me if that means its gold plated or is it a manufactors markings

18 k is the gold weight of the ring. It would not be gold plated with the gold weight on it.

Actually, plated jewelry can be stamped with the karat weight of the plating. 18k girlfriend can be plated with 18kt gold. Take the piece to a local jeweler and most can tell by looking at the piece through a gemscope