Merchant Discount Fees (or Merchant Discount Rate, MDR) are listed on your First Data contract under Section D: Price Schedule in the "Discount" column. They are listed as a percentage, and calculated based on the gross volume processed through the associated card type. For example, 1.99% x $1000 processed on VISA is $19.90 in fees.
You can also find your discount rate listed on the first page of the First Data monthly statement. The total discount fees for you can be found under the Service Charges column starting on page 3.