Check for Ajax request in CodeIgniter

I'm in a PHP script and I want to check whether the request is an Ajax request. (Basically to NOT allow direct script access, other than Ajax calls.)

So, I'm defining IS_AJAX somewhere in the main index.php file:

define('IS_AJAX', 
       isset($_SERVER['HTTP_X_REQUESTED_WITH']) && 
       strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest');

And then checking it at the top of my script:

if (!IS_AJAX) exit('No direct script access allowed');

Since I'm new to CodeIgniter, I would like to know:

• Is there any such built-in functionality?
• Is there a more elegant way to do it?