function isAjax() {
   return (isset($_SERVER['HTTP_X_REQUESTED_WITH'])
              &&
            ($_SERVER['HTTP_X_REQUESTED_WITH'] == 'XMLHttpRequest'));
}

http://www.gmarwaha.com/blog/category/client-side/javascript/

Quick and easy way to determine if the HTTP request is an AJAX call or a standard call.  Supports the notion of Unobtrusive JavaScript.

Check if request is AJAX request

http, ajax, header

PHP