Source

File: wp-includes/class-wp-customize-manager.php

public function doing_ajax( $action = null ) {
		if ( ! wp_doing_ajax() ) {
			return false;
		}

		if ( ! $action ) {
			return true;
		} else {
			/*
			 * Note: we can't just use doing_action( "wp_ajax_{$action}" ) because we need
			 * to check before admin-ajax.php gets to that point.
			 */
			return isset( $_REQUEST['action'] ) && wp_unslash( $_REQUEST['action'] ) === $action;
		}
	}