Well how can I tell if this is authentic so that I can determine what the value is on this?
to APF, rdavey.
The first place you should probably start is by measuring the poster. That particular poster (like Bruce's emovieposter.com's listing says) is a full 27x41 inches.
If it is slightly less, then you can start to be suspicious.
I personally don't know of this poster having been reprinted, but you can check
http://movieposterauthenticating.com/Movie%20Poster%20Authenticating.htmla website devoted to specific titles that are known to have been reproduced and the comparison photos of originals and reprints. Many of the same issues, like fuzzy text, inconsistent image cropping, etc., are covered in great detail there.
Also, take a close look at Bruce's scan of the movie poster and compare it against yours, to make sure all the fine type (copyrights, etc.) are the same as what you have on yours. Any differences are red flags that should be investigated further.
Additionally, you can help us help you if you post pics of the posters in question.
Let us know what you find out.