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.
Pure gold is not attracted to magnetic fields, but if an enormous magnetic field is applied to gold, the gold will slightly move and then slightly repel it. However, this is only so slightly and so no, it cannot be found with magnets.
Pure gold on its own does not magnetize or stick to a magnet. But when about 20% of it is replaced with a metal like iron, it may stick to a magnet or exhibit other magnetic properties. Gold used in jewelry may also be magnetic depending on the alloys used, i.e, the metals combined with gold in the material.
Gold, silver, and other metals are actually not magnetic. With that being said, certain spoof materials are, in fact, attracted to magnets.
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.
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.
By itself gold is not attracted to the magnetic fields we come across in our everyday lives. If you have a massive magnetic field then gold will be ever so slightly magnetic. It's safe to say that for practical purposes gold is not magnetic.
It can still be a low-purity solid gold alloy that contains magnetic metals. On the other hand, a piece may be gold plated, but it will not attach to a magnet because its core metal is non-magnetic.
The Magnet Test
Hold a strong magnet next to a piece of gold and watch for a reaction. Gold is not magnetic, so there should not be any attraction to magnets. If there is, you most likely don't have real gold.
Pure gold on its own cannot stick to a magnet. However, if you have an alloy of gold, then it could stick to a magnet. An example of a gold alloy that may stick to a magnet is gold with over 20% of its atoms replaced by iron. In very cold temperatures this alloy of gold may magnetize all on its own.
It is possible that an 18k gold piece sticks to a magnet. 18k or 18 karat means that a specific alloy contains 75% of pure gold and 25% other metals. Since pure gold is not magnetic, gold itself will not be responsible for the magnetism, but the metal alloyed may be.
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.
Drop the gold into a jug of water to see if it sinks.
Get a container big enough to hold both the water and the gold you'd like to test. The water temperature you use doesn't really matter, so lukewarm water is fine. Real gold is a dense metal, so it falls directly to the bottom of the jug.
Color and Appearance
First, they're both yellow metals. While brass is not bright yellow, it's more dull than gold. However, gold is much shinier causing its golden color. Impurities can cause the shiny metallic luster that is characteristic of gold to fade.
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.
Goldmeter - real gold detector on the App Store.
Gold is a relatively soft metal, and consequently gold coins that are made of pure gold or a high percentage of gold, can be relatively soft and malleable. Therefore, biting the coin would leave a mark or an indent if it was real gold.
Things You Should Know
Make a small scratch on the gold and add a drop of bleach. If it changes color, your jewelry is not 100% pure 24K gold. Bleach can permanently damage gold jewelry. It can't tell you if your gold is fake, since most gold jewelry has other metals mixed in to give it strength.