Every time you add a new product to the site, we mark it with an "Added to Site" label in greyed-blue text. You'll see this label right next to the product details, making it easier to identify already pushed products and determine if they need to be purged.
A product states it's already been added to my site; however, it's not actually on my site.
In this case, you may need to run a purge from the respective vendor, as the product is no longer available on your site.