How can I tell, by visiting a site, what web server is being used?
This gives you the HTTP Header, example output: HTTP/1.1 301 Moved Permanently Server: nginx Date: Mon, 26 May 2014 16:11:30 GMT Content-Type: text/html Content-Length: 178 Connection: keep-alive Location: http://ycombinator.com/
$ curl -I www.reddit.com
HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
x-frame-options: SAMEORIGIN
x-content-type-options: nosniff
x-xss-protection: 1; mode=block
Server: '; DROP TABLE servertypes; --
Date: Mon, 26 May 2014 18:37:12 GMT
Connection: keep-alive
Vary: accept-encoding