Firstly, identify gold plated jewelry by content marks.

Find identification marks on the jewelry. HGE is a common marking for gold electroplate. This is a gold plated jewelry. So is RGP or rolled gold plate, a mark often seen on vintage jewelry. Any gold overlay is gold plated jewelry, including vermeil (pronounced ver-may). Vermeil is gold plating over a sterling silver base, and this is fine jewelry, although the gold plating may wear over time.

Secondly, look for costume jewelry maker marks.

Know the costume jewelry maker names and look for these. Names like Napier, Robert, Kramer, Monet are not likely to be gold plated. Costume jewelry is often a base metal with a metallic coating that is not 9k gold or above, and probably is not gold at all. This is called goldtone jewelry.

Thirdly, look at the corners and edges.

Check the jewelry with a loupe to see if there are worn areas on the corners or edges. This will frequently identify gold plated jewelry, because the top layer wears on the corners and edges, and reveals a different color or type of metal under that surface.

Finally, test with an electronic gold tester.

Test questionable items with an electronic gold tester if they are not marked by content or maker. This will accurately identify gold plated jewelry, and the acid test kits will not. Acid test kits recommend scratching the jewelry to get past the surface layer, but good gold plated jewelry often tests gold although it is not. Identify gold plated jewelry before you buy--since it has lesser value than gold filled or karat gold jewelry.