How can you tell if an earring contains Nickel?

One good way to tell is to use a nickel test kit. This is practical when you want to test an earring which you've already purchased. If you are purchasing a new earring, look for "guaranteed nickel-free" or "hypoallergenic" or "100% nickel-free". Jewelry marked with one of these will have no nickel or very little nickel and are less likely to irritate.