Source

File: wp-includes/load.php

function wp_is_jsonp_request() {
	if ( ! isset( $_GET['_jsonp'] ) ) {
		return false;
	}

	if ( ! function_exists( 'wp_check_jsonp_callback' ) ) {
		require_once ABSPATH . WPINC . '/functions.php';
	}

	$jsonp_callback = $_GET['_jsonp'];
	if ( ! wp_check_jsonp_callback( $jsonp_callback ) ) {
		return false;
	}

	/** This filter is documented in wp-includes/rest-api/class-wp-rest-server.php */
	$jsonp_enabled = apply_filters( 'rest_jsonp_enabled', true );

	return $jsonp_enabled;

}