You will get some information from the Server HTTP header. For example, our web server shows
Server: Apache/2.2.9 (Debian) DAV/2 SVN/1.5.1 mod_jk/1.2.26 PHP/5.2.6-1+lenny3
with Suhosin-Patch mod_python/3.3.1 Python/2.5.2 mod_ssl/2.2.9 OpenSSL/0.9.8g
mod_perl/2.0.4 Perl/v5.10.0
which you can check with using a website like SEOConsultants' Check Server Headers Tool or a program like netcat:
yourhost:~% nc someserver.com.au 80
GET /asdf HTTP/1.0
Host: someserver.com.au
HTTP/1.1 404 Not Found
Date: Wed, 28 Oct 2009 11:49:27 GMT
Server: Apache/2.2.9 (Debian) DAV/2 SVN/1.5.1 ...
Last-Modified: Tue, 27 Oct 2009 12:49:03 GMT
Content-Type: text/html
Connection: close
<html>
...
Note that this information may be obscured or replaced by server administrators.