What is the easiest way to find out the value of baseball cards without having to become a paying member of a site?

Look up your card on eBay.

Book values are nothing more than a guideline.

The real value is what a willing buyer is willing to pay for the card, regardless of book value, so eBay will give you the best information if someone else is selling the same card.

You could also click on the following search. You will find many sites where you can find the possible monetary values without having to pay anything.

Baseball Card Values - Google Search

And, in agreement with the above answer, the book values are only a guideline. People who collect collectible cards get really picky about the condition of the cards concerning the possible monetary value of them. So, it's best to be taking cards to an expert who can see the card or cards in person in order to arrive at a possible monetary value.