Or, if you want a practical answer: There are books that can tell you the value of comics, depending on what condition your copies are in, and services that will actually rate your comics "officially" and sometimes seal them in protective cases. The only book I'm actually familiar with is the Overstreet Price Guide, which you can find on Amazon or any other bookseller website. Can't remember the names of any of those rating services, since I don't really bother with that myself.
Also, it might be worth talking to the owner/manager of any local comic shops in your area. My shop's owner will often refer to the price guides only roughly, take a look at the comics for himself, and negotiate directly with people.