Source

File: wp-admin/includes/network.php

function network_domain_check() {
	global $wpdb;

	$sql = $wpdb->prepare( 'SHOW TABLES LIKE %s', $wpdb->esc_like( $wpdb->site ) );
	if ( $wpdb->get_var( $sql ) ) {
		return $wpdb->get_var( "SELECT domain FROM $wpdb->site ORDER BY id ASC LIMIT 1" );
	}
	return false;
}