are these links intended to be on pages outside the cart? If so, you could just add a new get parameter to the url, like ...index.php?main_page=product_info&products_id=319&source=1, and on the other link you could have ...index.php?main_page=product_info&products_id=319&source=2.
The cart will just ignore that extra get parameter, but you could see it in the access logs.

You could build some custom code to track the source parameter within the cart (it could go in includes/modules/pages/product_info/header_php.php)