Some of the signs that a necklace is real gold is if it does not float or does not stick to a magnet. Owners can check the markings on a necklace or take it to a jeweler for testing to determine if it is real gold.
Many necklaces have a hallmark, which is the symbol of the manufacturer, along with the purity stamp, which tells the purity of the gold in the item. Such marks are usually on or near the clasp. Not all jewelry will contain a hallmark, since they are used primarily by countries that require a Common Control Mark. Gold pieces manufactured before 1900 may not have a stamp, since it was in that year that the U.S. government required the purity stamp. The mark is usually small and requires a magnifying glass or jeweler's loupe to see it.
Gold is not magnetic and as such, a gold necklace should not be attracted by a magnetic surface. However, the necklace could be gold plated and the magnet may attract the metals beneath the surface. Real gold also sinks in water and does not become rusty or discolored.
More detailed tests, like the acid test, require the assistance of a jeweler who is a trained specialist. He or she has the tools to scrape the necklace without damaging it, along with the proper chemicals to determine whether the piece is gold. A jeweler can also test and weigh unmarked necklaces to reveal the purity content.