Primary menu hook

I’m trying to use this code:

add_filter('wp_nav_menu_items', 'add_login_logout_link', 10, 2);
function add_login_logout_link($items, $args) {
if( $args->theme_location == 'top-menu' ) {
    ob_start();
    wp_loginout('index.php');
    $loginoutlink = ob_get_contents();
    ob_end_clean();
    $items .= '<li>'. $loginoutlink .'</li>';
}
return $items;
}

However I cannot work out what my main menu is called. It is not ‘top-menu’ as per the above. What is the name of this menu? Thanks