You can test each one:
Code:
$chk_xyz = $_SESSION['cart']->in_cart_check('products_fornitore', 'xyz');
I use the variable $chk_xyz to match the thing I am looking for just for clarity and so I have a count for that particular number of Products in the shopping_cart for the tested value ...

I could use $fred as the variable but that isn't very clear in the code ...