Things You'll Need
Instructions
Decide which pieces of jewelry you may think are made with a precious metal, such as gold.
Spread out the pieces you have selected on a flat surface so they are not touching.
Take your magnet and gently place it right above each piece of jewelry. If the piece is attracted to the magnet, it is not made from a precious metal and is costume jewelry. If it has no reaction, you have just found a piece made from a precious metal.
Look closely at the clasps or earring backs on each piece of jewelry. If they look uncommon, the piece is most likely old. Clasps years ago include the "c" clasp, in which the pin fits into a half metal loop that closes to secure the pin in place. Old earrings had shepherd's hook backs or screw backs.
Use the Internet to look through old advertisements for your type of jewelry. This is a common practice to gauge the age of pieces--plus, it is fun.
http://illusionjewels.com/costumejewelrymarks.html
http://www.emcity.com/ads.htm