Way to tell if a post came from an ajax call in codeigniter?

I just started using CodeIgniter after using Zend for a while. My new site has a feature where you register through Ajax. In Zend I could use this to check if the incoming POST was through AJAX, and therefore from my site:

if(!$this->getRequest()->isXMLHttpRequest())

Is there a piece of code in CodeIgniter that does the same thing? If I don't make sure it's an AJAX call, someone could theoretically register anything they wanted by creating a form to post to my controller.

Thanks!