Marbles are not just a childhood plaything. Many of the American marble-manufacturers were put out of business in the 1950s, which means the marbles they produced are now considered rare. That means marbles made before 1950 may be worth a considerable sum to serious collectors. If you have found a bag of your grandfather's old shooters, it could be worth your time to know what to look for to determine their value.
Place all of the marbles into one pile. Separate all of the solid-colored, clear or Cat's Eye. These are essentially worthless because they're too common to have any value of the secondary market.
Pick out any cracked, scratched or damaged marbles. Collectors are unlikely to want a marble with visible damage, which makes these essentially worthless too. The exception would be an extremely old and rare marble. If you think you have one, even with a scratch or two, set it aside for future appraisal.
Hold on to the remaining undamaged and uncommon marbles. These are the marbles that could have some value. Take photos of just those marbles and send them to websites like BuyMarbles.com, which can appraise their value.