To check individual books, you can go to a used-book site like abebooks.com and do an advanced search, changing the order of the listing from "cheapest first" to "most expensive first." Then you can see the high end of what your book is worth.
You can also see how many copies are available, which gives another indication of its rarity.
If you are really interested, you can track a book over time to see whether its value is going up or down, which can tell you whether to buy, hold, or sell.
There is a lot to know about book collecting: the condition of the book, which printing it is and how to determine that, whether it's signed, British or American edition, etc. All affect value. The jacket doubles the price, usually.
BUMP!