Illusions
 

Is there some sort of contested roll for an intelligent creature (IQ 10 Werewolf for example) to figure out that the illusory wall of whirling blades (for example) are in fact illusory instead of some sort of advanced conjuration?

Re: Illusions
 

Considering that Know Illusion is a regular spell one would cast at the blades, there probably isn’t a simple “disbelieve” saving throw.

However, I’d certainly let someone roll Hearing to realize a simple illusion lacks sound, or Navigation to figure out the mechanism couldn’t possibly be in that wall because we’ve just been around the other side, and so on. This will give suspicion, enough to justify casting Know Illusion (or throwing a rock at it, really), but not make it go away.

Re: Illusions
 

Quoting the Spells book, description under Simple Illusion...

Any IQ 6+ being can dispel a Simple Illusion by “disbelieving”: taking a Concentrate maneuver and making a successful Will roll.

Re: Illusions
 

Sure, the issue I see is that they have to know to make the roll.

Re: Illusions
 

DFRPG Spells doesn't offer this, but my standard method is to roll a Quick Contest of IQ (not spell skill!) against the Per of anyone in a position to notice (save time and roll once for a group of foes with the same Per). If you win, the illusion fools that individual - and you make a fresh roll any time they interact with it. Roll a new contest each time someone you've already fooled changes how they're interacting with the illusion.

(The well-read will notice I cribbed this from GURPS Powers p.94, which does a much better job of outlining 'how to interact with illusions'.)

Re: Illusions
 

I'd allow a Thaumatology or Occultism check