In a view of products, add the word "Added" to all products already added to the cart
Sorry for the late answer, I meant to answer this, but for some reason got lost and today this Q just randomly popped into my mind. 

If you think creatively, you can accomplish this using CSS like this:

1) Create an attachment clone of your cart, by clicking the +Add button and selecting attachment. Remove all fields and add a unique field to it, such as SKU. (Make sure you use For: this attachment override, so you don't affect your actual cart view). 

2) Rewrite your SKU field to be like this

&lt;style&gt;
.[field_sku]:before { content: "Added"; position: absolute; } 
&lt;/style&gt;


3) In in the header section of your products view, add the attachment clone of your cart.

4) In your Products view, wrap each product inside a div class that has the SKU#

&lt;div class="[field_sku]"&gt;...&lt;/div&gt;


You could try doing the re-writes through the Views UI, else you can use a template file (hopefully you know how to use template files in Views).