The Rewards list view is automatically filtered to only include Rewards in the status of Pending, Signed Off or Approved. 

To find any historical Rewards that have been paid go the filter bar and press select the 'Status' filter

A drop down list will appear where you can select 'Paid'.

The list will now show all of your historical rewards that have been marked as paid.

Need to mark an approved Reward as paid but unsure how? Follow this article.