The date on the coin is going to be the best way to tell if a circulating U.S. Quarter is silver. The U.S. Mint first made quarters in 1796. From that time up until 1965, quarters were made with some degree of real silver. Since 1965, U.S. quarters have been considered “clad,” which means that they are coated. Their core is made of copper and is coated with a blend of copper and nickel.
If you find a circulating quarter that is dated before 1965, then it is made with some degree of silver. If the quarter predates 1935, then it is made with .1808 oz. of silver. Circulating quarters made from 1935 to 1964 are 90% silver.