One method you can use is:
<?php
// bof: get product views
$products_viewed_raw = "select p.products_id, pd.products_viewed from " . TABLE_PRODUCTS . " p, " . TABLE_PRODUCTS_DESCRIPTION . " pd where p.products_id = '" . (int)$_GET['products_id'] . "' and p.products_id = pd.products_id and pd.language_id = '" . (int)$_SESSION['languages_id'] . "'";
$products_views = $db->Execute($products_viewed_raw);
echo ($products_views->fields['products_viewed'] > 0 ? '<li>' . 'Views: ' . $products_views->fields['products_viewed'] . '</li>' . "\n" : '');
// eof: get product views
?>
Another method is with a function:
<?php echo ( zen_products_lookup((int)$_GET['products_id'], 'products_viewed') > 0 ? '<li>' . 'Views: ' . zen_products_lookup((int)$_GET['products_id'], 'products_viewed') . '</li>' : '') . "\n"; ?>