All I can suggest really is to start by calling a store that caters to comic book readers, often, the cards and the books go hand in hand, when being sold. But, don't stop at one shop if there is more than 1 in your city, call around to get the best idea. Hope this helps, good luck.
A way to get an approximate value of what it is selling for, go to e-bay and see. Another way is to go to a store that sells Beckett, and check the price in there. You didn't say what brand you had, but Thomas Rookie cards were REALLY hot when it first came out, but now the value is probably low. If you check Beckett, don't pay attention to the "Hi" value....that is what a dealer would sell it to you for....Look at the "Low" value to get a better idea of what it's actually worth. Good Luck