Lay the comic book on a flat surface. Open the comic book to the first page.
Look at the bottom of the inside front cover (IFC) and locate the publishing indicia, affectionately referred to as "legal mumbo jumbo," which lists the copyright and any legal disclaimers. In rare cases, the indicia is on the first page, or near the back of the comic, however, the convention is to place it at the bottom of the IFC.
Note the publishing date, listed in the indicia; this denotes the publication date of this particular issue. The publishing date often accompanies the print run information - which lets you discern if this is a first, second or third printing -- and determines the value of the book, as only first printings usually carry any value over and above cover price.