Using genesis_site_layout()

public function content_add_class( $attributes )
    {
        // Default Genesis layouts
        # content-sidebar
        # sidebar-content
        # content-sidebar-sidebar
        # sidebar-sidebar-content
        # sidebar-content-sidebar
        # full-width-content
        $layout = genesis_site_layout();

        if ($layout !== 'full-width-content') {
            $attributes['class'] .= ' col-8';
        }
        $attributes['class'] .= ' col-12';

        return $attributes;
    }
add_filter( 'genesis_attr_content', array($this, 'content_add_class') );