I just came across a box full of old RPG books and modules that are in great condition. How do I find out if they are worth anything?
PoD (Print on Demand) is becoming more common and that will affect the number of people buying the old out of print RPG books. As example, WoTC on drivethrurpg.com is offering a slew of old AD&D stuff as PoD. Here's a youtube review of one with a side-by-side comparison.
While that will not affect the true collectors, it will cut out all the people that want the books just to use them.
As for price, it's basically what the market will bear. There are no universal firm listings like with comic books but some folks try to do that, see Rulebooks Index. You can get better ideas on price by looking at past and current prices on sites like ebay or amazon. Both sites have lots of second hand books for sell.
I'll list a few for AD&D but you should Google for whatever games you have.
Or you can look at this other question, it had some good information and links to resources.