There is a book called Beckett. It is what is know as the Sports Cards Collector's Bible. They have a website www.Beckett.com
The Beckett has online price guides as well as you can buy the Beckett at Toys R Us Stores, Walmart and Target in the section that sell Magazines. You will have to pay for a Beckett or you can subscribe online by the month for the Football section.
The last way is to check Ebay.com Type in the year of the card and the players name and you should get a list of cards that will give you an idea of card value. The prices will vary form dealer to dealer, but will give you an idea of card values. Joan