You can use Ebay Completed Listings to check the prices of cards.
Just search the card name, then towards the left there should be a box that says Completed Listings. Click that, then once it searches again, find the scroll down in the upper right - select "End date: recent first". It should search again. Then, what I do is I take the prices of the last 5-10 items I sold (sold items are green not red) and average them to get a good idea of the cards value.