Get a ceramic plate and scratch the gold piece you're testing against it. If the gold is real, you should see a yellowish streak on the plate. If it's fake, you'll see a dark streak.
Vinegar is very destructive, so when you apply the vinegar to a fake piece of gold, the vinegar will cause the jewelry to tarnish. Gold is a non-reactive metal, so real pieces of gold will not react to vinegar.
Fill a cup or bowl with water and carefully drop your gold piece into it. If the gold is real, it will sink to the bottom of the cup. If it's fake, it will float to the top or hover in the middle of the cup.
The Skin Test
One of the easiest methods of testing gold purity is the Skin Discoloration Test. Real gold won't leave any stains or spots on your skin. However, if you are wearing fake gold jewelry or handle fake gold bullion - your skin will discolor. Just hold your gold item in your hand for a couple of minutes.
Scratch test.
If you cut deep enough that you expose underlying metal, you can assume that it is plated. If it appears to be made of a uniform composition throughout, then it is probably solid gold.
The Ceramic Scratch Test
Take an unglazed ceramic plate or piece of tile and scrape a piece of gold across its surface. Real gold will leave a gold mark or trail. Other metals will leave a black trail.
Some types of gold, such as gold-plated items and gold that has been alloyed with other metals, may be attracted to magnets. This is because these types of gold have a higher concentration of electrons, which makes them more susceptible to magnetic attraction.
If it's real gold it will not stick to the magnet. (Fun fact: Real gold is not magnetic.) Fake gold, on the other hand, will stick to the magnet. If that necklace leaps to the magnet, your significant other has some explaining to do.
Goldmeter - real gold detector on the App Store.
Neodymium magnets are very strong and can help to test the authenticity of gold and silver bars, coins, bullion, or jewelry. However, it is important to note that this method is not foolproof.
Rub your gold against a jeweler's stone.
Position a black jeweler's stone on a table. Hold your gold piece firmly in your hand. Wipe it across the stone firmly enough to leave a mark. If the mark that you've left on the stone is solid and gold in color, then the piece is pure.
The Acid Test
Make a tiny mark on the piece of gold to penetrate the surface. Drop a small amount of liquid nitric acid on that scratch and wait for a chemical reaction. Fake gold will immediately turn green where the acid is. Gold-over-sterling silver will become milky in appearance.
Pure gold (24K or 999) has a rich, nearly orange-yellow colour, however gold rarely occurs in nature in pure form. It generally grows as a crystal structure including other elements, like silver or copper.
Fool's Gold
It's a form of pyrite that is almost always mistaken for gold due to its brass-yellow color. Its surface is rough and made up of crystals. There is no value to fool's gold, but it looks like genuine gold at first glance.