Turn the piece of jewelry over. There should be markings or stamps on the bottom. Use the jeweler's loupe to magnify the area so that you can get a good look at the stamped markings. If the back side of the jewelry is dirty, making it difficult to read the markings, use the white part of the silver-polishing cloth to gently rub the area slightly cleaner so you can read the markings more easily.
Write down the words, symbols and marks that you see on the jewelry. There is often a small bell-shaped stamp with a number inside it. This is an important identification item in Mexican silver jewelry and is known more formally as an "eagle hallmark." Often stamps and hallmarks are worn away or are incomplete.
Look in one or more book about Mexican hallmark identification to find information about the piece of silver that you are trying to identify. The books will give you an idea of when the piece was made and possibly its maker. Common identification marks for authentic Mexican silver jewelry include an eagle stamp and the words "Hecho en Mexico."
Use the silver acid-test kit and stone to rub or scratch out a tiny line of silver (about 1/32 inch wide) from the jewelry on to the stone, in an inconspicuous area. Following the instructions, apply a small amount of the silver acid and wait to see whether the piece you have is genuine silver. If it is genuine silver and not plated, it will turn a cloudy, pasty color or have a greenish tint. Because many Mexican silver jewelry pieces have gemstones, shell, enamel, or mixed metals in them, specific-gravity tests and magnet tests may not yield accurate results. More details on acid testing can be found at The Sterling Silver Guide (see Resources).
Polish the piece with the silver-polishing cloth--but keep in mind that polishing removes a fine layer of silver and often is not recommended for vintage jewelry.