FC should add an item that works like this: when you right-click on it, it removes all your buffs and also displays a text above your head (in the same way so many other items do). Something like "There goes all my buffs"
Two persons agreeing to duel self-buffed could then use this item, rebuff with their own buffs, and both would be certain the other one is self-buffed.
The item should be available in one of the basic shops, just like a key duper or lockpick.