Well, first off, there IS a way to tell how rare it is by looking at the card. A circle indicates a common card, a diamond is an uncomon card, and a star is a rare card. Start by looking for these.
A good place to check for card prices would be Ebay itself. Just look at older auctions of the same card(s) and you can average out different prices.
Here's a website that actually helped me. It takes you step by step, and I prefer the second method.
Hope this helps =)