Are there actions or filters I can use for Ajax calls?

If you check the source for admin-ajax.php you will notice that the admin_init hook fires and that the system checks $_REQUEST['action'] for hooks. Given that, you could hook to admin_init, check the “request” super global, construct wp_ajax_* and wp_ajax_nopriv_* values, then use has_action() to see if anything is hooked in.

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