This is an area that I found out about a while ago while I was at a local comic book shop. This actually applies to the front cover that is on comic books, and can be a fun area to dive into.
When a publisher like Marvel, IDW or DC puts out a comic, they can give it another cover to help commemorate a special event like a new character coming about or it being the first issue of the series. Because of this, collectors will more often than not pick up two issues to get the different covers.
How do you know when you have an alternate cover on a comic book?
One way to help to you out is to look at the corners of the cover.
This will provide you information on who made the comic and issue number. If this is the same, then you stand a good chance that you have the alternate cover. Another thing to remember is that sometimes the comic producer will put an “A” or “B” after the number to help with identifying what the comic is.
Another place to look is at the copyright information on the bottom of the first page. With comics, the producers will change the volume number if they decide to either do a reboot of the series and start fresh, or if a totally new chapter is starting with the series (like what Marvel did with the HEROS REBORN stories that took place with several titles several years ago). If the copyright information is the same in both books, then you are one step closer.
The last thing to do is to flip through both comic book itself. What you are looking for is to see if both books are the same all the way through (the covers on both of the books will be the only thing that will be different).