Is Ajax in WordPress
Update: since WordPress 4.7.0 you can call a function wp_doing_ajax(). This is preferable because plugins that "do Ajax" differently can filter to turn a "false" into a "true".
Original answer:
If you're using Ajax as recommended in the codex, then you can test for the DOING_AJAX constant:
if (defined('DOING_AJAX') && DOING_AJAX) { /* it's an Ajax call */ }
WordPress 4.7 has introduced an easy way to check for AJAX requests, so I thought I would add to this older question.
wp_doing_ajax()From the Developer Reference:
Description: Determines whether the current request is a WordPress Ajax request.
Return: (bool) True if it's a WordPress Ajax request, false otherwise.
It is essentially a wrapper for DOING_AJAX.