The easiest way would be to just add a column to your table to indicate a file has been downloaded. If you can't modify that table, you could create another one or even use a flat file to keep track of your downloads.
The contents of the table are going to be recreated in whole each day as well. Meaning I am going to truncate and populate the tables prior to the ftp - so managing this information within the table probably wouldn't work. However, maybe a secondary table would do the trick.