There are better ways to do this, but let us assume you have no access via SSH or even FTP to be able to add a file.

Linux / OSx

Open up a terminal window and type the following;

curl --head http://thesite.com

This will return something similar to:

HTTP/1.1 200 OK
Date: Wed, 15 Aug 2012 10:41:32 GMT
Server: Apache/2.2.9 (Debian) PHP/5.3.8-1~dotdeb.1 with Suhosin-Patch
X-Powered-By: PHP/5.3.8-1~dotdeb.1
Set-Cookie: PHPSESSID=47d6a5e22c54307f211f67294f17495f; path=/
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Keep-Alive: timeout=15, max=100
Content-Type: text/html
Accept-Ranges: none
Connection: Keep-Alive

And as you can see in this information you can find the Apache and PHP information.