Testing kits include various bottles of nitric acid for different types of gold. Some tests also include a flat rock called a touchstone you can use to scratch off some of the gold on your item. You may also see needles with samples of yellow and white gold to use as a comparison to your item. Testing kits are available online. Also, check with local jewelry stores. Most jewelers use this test for its accuracy. Choose an inconspicuous location on the jewelry to make the scratch, such as underneath a clasp or an inner band. Then, use a sharp tool such as a jewelry engraver to dig into the gold. Scratch until you get underneath the top layer of gold. Expose a fresh layer of gold or any other metal underneath it. Nitric acid tests require you to scratch your piece of gold. If the gold has personal value to you or you plan on keeping it, take it to a professional jeweler instead of doing the test yourself. Put on latex gloves and work in a well-ventilated room to avoid any issues with the dangerous acid. When you’re ready, look for the bottle of acid labeled for 18K gold. After setting the gold in a stainless steel container, place a drop of the acid directly on the scratch you made, then watch for it to turn a shade of green. If it turns green, you know right away your gold is fake.  Regular gold does not react to the acid, so your item might be gold plated or a low-purity blend of metals. A milk-colored reaction usually indicates gold-plated sterling silver. If the acid turns gold, you have gold-plated brass. If you think you might have real gold, rub it along the touchstone to create a streak of gold flakes. Add a drop of 12K, 14K, 18K, and 22K nitric acid to different parts of the streak. Check back after 20 to 40 seconds. Find the spot where the acid doesn’t dissolve the gold to figure out what karat rating your item is. The acids all increase in strength, so the acid used for 22K is stronger than the 12K one. If the 18K acid dissolves the gold but the 14K one doesn’t, you know your item is probably around 14K.
Purchase gold-testing kit to get the acid you need for the test. Create a small scratch on the gold using a sharp tool. Add a drop of nitric acid to the scratch. Scratch the gold on a touchstone to test its purity.