Gold plated jewelry is 9k gold or higher, and may or may not be marked, depending on whether it is made in a studio, or in a large manufacturing plant. Gold filled jewelry was made in the 1950s and 1960s primarily, and it has more gold content than gold plated jewelry, and is often marked with 1/20 12k gf or similar marks. Gold plated jewelry has a layer of gold on the surface, and those who test gold with an acid test kit often find that gold plated jewelry will test gold, but is not.
Know the difference in these three categories when purchasing gold jewelry. There is a difference in value.
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.
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.
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.
Test with an electronic gold tester.
Test questionable items with an electronic gold tester if they are not marked by content or maker.