Source

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

public function is_cross_domain() {
		$admin_origin = wp_parse_url( admin_url() );
		$home_origin  = wp_parse_url( home_url() );
		$cross_domain = ( strtolower( $admin_origin['host'] ) !== strtolower( $home_origin['host'] ) );
		return $cross_domain;
	}