Although most sports card collectors are usually fans of the game as well, some are actually into the card collecting hobby for the potential profit they may reap from the trading. Therefore, it certainly doesn't hurt to know the methods that you can use to assess this, even if you've no intention to sell off your collection now.
Here are 3 simple and effective ways commonly used :
(1) Ebay Auctions
A relatively fast way to know how much people are willing to pay for your particular card is to do a search on Ebay auctions. While doing so, it's vital to go by what buyers are offering instead of the price sellers are asking for.
Even though both demand and supply has the same important part to play, looking at the buyers' market is the fairer way to determine the current "market value". The number of bids, depending on the base price set by the seller can also help you determine if the card you're holding is "hot" or not. That in turn can help you plan your own baseline selling price next time.
(2) Monthly Magazines & Price Guides
If you're more interested to know the "book value" for your cards, depending on the amount of money you want to spend and the depth of the details you want to know, there are avenues for you to check that as well.
For a start, you can check this via the monthly trading card magazines. Such periodicals tabulate current book values of the most popular range of single cards and sets and can bought off newstands at about $4-5 per copy. 2 of the main magazines are Beckett and Tough Stuff.
Suppose you've got a bigger budget, go for the $10 paperback or the $40-50 price guide. Both give you value estimates based on the production year of the sets from different different card companies with the latter covering on almost every set that are issued for all major sports.
But regardless of which option you choose, do bear in mind that such value estimates are highly subjected to the condition and maintenance of the cards concerned. So, in case you don't get the price that's stated on the periodicals, don't go back to your newstand vendor or book store and start asking for a refund!
(3) Trading Card Clubs & Forums
Last but not least, just go asking around. By doing a search on Google, you'll be able to get a listing of all online card traders clubs and forums you can join and learn about such knowledge. Members are often able to give you quotes on the spot and who knows? You might even get a good offer for your card. Besides, it's also gratifying to know a bunch of people who share the same interest as you, isn't it?