There are multiple ways to discover the value of comics, but the simplest is to purchase a comic book price guide. A good guide, such as the Overstreet Comic Book Price Guide, lists all major comics titles and what they are worth, based on the condition the title is in.
Checking the grade or condition of a comic is best done by professionals, who examine comic books regularly and know what to look for. Many local comic store owners and workers are more than happy to assist in grading comics, though there are also professional comic graders who will grade and certify comics for a small fee.
After checking a comic book price guide and grading the comics for sale, it is worth browsing online comic stores such as Mile High Comics and auction websites like eBay to see how similar comic books are selling currently, as this may differ from the comic book price guide.
Many factors such as original print run and current comic story lines will affect the overall value of comic books. With so many factors involved it's important to consult all the resources available to determine the current value of the comic books, including local fans and experts if available.