FADEC, not sure if it is possible to do the check whilst you are doing the wash run - with both engines at idle but...
...I would recommend that it is done before flight - or at least before the first flight of the day - to ensure that it is working following any maintenance done on the aircraft; it might be that the system was disturbed during work carried out on the aircraft and finding out it's not working when you really need it might have you wishing you spent the two minutes it takes to check it.
Wiz