Quite simple really, let the Arsonist know who is doused at night by painting their buttons orange or something.
Several reasons for this:
1. Witch can make arson douse someone he didn't intend, thus his track of who he doused is incorrect and may result in his loss.
2. Bus Drivers, same effect as Witch.
3. Other Arsonists! lol
Yes Arsonist should be able to see who is doused at night, even if people were doused by another Arsonist. He is an expert on gasoline after all, so he knows where the gas is. Meaning he should know if he was doused himself (by witch/bus driver making him do that, or another arsonist).
How hard to do would this be...?