It really depends on what type of card you have.. You can look your card up in the Beckett's catalog or on their website or you can look on EBay for a similar card that you can compare it to.. Your best bet is to take it to a reputable sports card/memorabilia shop and let them take a look at it... They'll be able to place a value on the card.. Usually they will look it up on EBay anyway... Good luck.....
When the memorabilia market was booming, football cards were worth a fraction of baseball cards. And since the crash, the inflated prices have hit rock bottom -- unless it's very rare. Get it appraised by a reputable dealer in your area, but you paid a buck for it for a reason....and it is not because of the uniqueness of the card.
How should we know? You'd need to get it graded first and foremost, and when you find out that it is FAKE you won't think that deal was so great probably.