If you aren't running OE, then you can download the unit's punches using SI-Config.
Open a download unit with SI-Config and put the unit to be interrogated upside down on the unit (use the coupling stick if you have one, otherwise just line the holes up).
Read the unit in SI-Config.
Use the menu command Commands>Read Out Station.
You should see a list of all the cards that have been to that station since it was cleared.
If you can't spot the competitor's SI number quickly, save the file using the menu command Commands>Ascii interface, then open that file in another program like notepad or even Excel and use the Find command to see if the number is there.