Instructions
Look for a shine. The silver alloy nickel is shinier than standard nickels, as silver holds the mint luster better than other metals. Hold several nickels in your hand to tell if one has this particular shine.
Look at the dates. If the Jefferson nickel does not have a date between 1942 and 1945, it cannot be a silver alloy nickel because they were only minted during those years.
See if there is a large "P," "D" or "S" mintmark above the dome of Jefferson's home of Monticello on the reverse of the coin, indicating they were minted in Philadelphia, Denver or San Francisco. Mintmarks for other alloy combinations were stamped to the side of the Monticello image instead of above the dome.
Protect your coin. If you find a silver nickel among your change, put it in a snap-tight plastic holder or a cardboard sleeve that folds over the coin that you can staple together. Silver alloy nickels that grade only very fine (showing some wear, but with most features of the coin still defined) are worth around $1 to $1.50.