How do I see the Messages that have been sent?

To review the Messages that have been sent out from the Opportunity you will want to look at the Email History list found within the Email Communications tab. 

From here you can see the entire list of the Messages that have been sent out at the Organization level, as well we all Opportunities. The main list view provides an overview of each Message, and there is also a details pop-up that will provide more specific details on the recipients, deliveries, and bounce rates. From the detailed pop-up, you are also able to preview the message that was sent.