[UnrealEd]

I used a small script that attempts to connect to the website through a socket. I simply set the default timeout of the access-attempt to 10 seconds, and when the timeout was reached, I stated the url as invalid:

PHP Code:

/**
 * Checks if the entered url links to a valid website. 
 * A valid website is one that responses within a timelimit of <code>$seconds</code>
 *
 * @param string The url that need to be checked
 * @param integer The number of seconds before a timeout occurs. default=10
 * @return boolean True if the website responded, false otherwise
*/
function isValidWebsite ($url, $seconds=NULL) {
  $realUrl = preg_replace ('#https?:\/{2}([^\/\\]+).*?#s', '\\1', $url);
  $seconds = is_null ($seconds)? 10 : $seconds;
  return fsockopen ($realUrl, 80, $errNo, $errStr, $seconds);
} 

There might be some easier ways, but this one has worked fine for me