has_post_series && get_post_series

part of GWP tutorial : https://generatewp.com/group-post-series-custom-taxonomy

    /*
     * has_post_series 
     * used to determine if a post is a part of a post series
     * @param  int  $post_id  post ID
     * @return boolean        true if the post is a part of a post series
     */
    function has_post_series( $post_id ){
        return has_term('', $this->tax_name, $post_id);
    }
    /*
     * get_post_series
     * Get post series term object by post ID
     * @param  int $post_id post ID
     * @return WP_Term|boolean  series term object if found or false if not.
     */
    function get_post_series( $post_id ){
        $series = wp_get_post_terms( $post_id, $this->tax_name );
        if ( ! is_wp_error( $series ) ){
            return $series[0];
        }
        return false;
    }