Affiliate referrals are listed for each order in your Transaction Log:
http://www.e-junkie.com/ej/help.transaction-log.htm
You could just scan across your log visually to get a rough idea of how many orders had affiliate referrals vs. not. For a more precise calculation, you can select a date range and click View Log, then click Download Log to export that log view to a file you can unzip and open in a spreadsheet program like Excel; there, you can sort rows by the Affiliate ID column to group affiliate sales together, then compare the number of rows with Affiliates vs. those without or calculate sums to compare.
