FASA Games, Inc.

Thinking through this, something akin to what The Undying has written. If the players or PC's have no reason to suspect an illusion, the GM could roll their Per (or other stat) against the Sensing Difficulty, then tell them, "Hey, that spell hurt, but something was off about it." Introduce illusions this way. Or if it does more damage than its Sensing difficulty, or the effect tests was more, or whatever. Just some trigger that indicates to the GM they should give the players a chance.

And they should definitely be allowed to disbelieve any attack spell. Well, try to, anyway.