First, to find out if a piece of jewelry is made of 18-carat gold, check for the presence of a hallmark. Indeed, the hallmark is an official mark used by jewelers to identify the provenance and purity of a piece of gold jewelry. Second, you can use a magnet. If your jewelry is not attracted to the magnet, it is made of gold.
For any other questions, you can always ask for Artémi advice. Having more than 20 years of experience, she will be able to determine if your jewelry is made of gold.