Most of the times pages are named like index.php or page.aspx and you know what language was used.
But what about something like this page :
http://www.ruby-forum.com/forum/ruby
Is there anyway to find out which server side scripting language are they using?
The backend language of a web-site is information which usually should not be available to the client. That many languages reveal themselves via their file extension is a lucky coincidence for you. So I think, the answer is: No, in general that is not possible. Although you might be able to think of a set of heuristics (e.g., in your example above, you could "guess" ruby).