You can have a look at the headers of the server response, however the information will not be complete, and might even be bogus.

Anyhow, the Server header looks something like this (tried on perl.com):

Server: Apache/1.3.26 (Unix) PHP/4.2.1 mod_gzip/1.3.19.1a mod_perl/1.2
+7
[download]

...so you can guess it's Unix in this case. This is the header for perl.it:

Server: Apache/1.3.27 (Win32)
[download]

Hope this helps!

Michele.