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"A character faced with proof that an illusion isn't real needs no saving throw."
edit: There's no RAW way for Spellcraft to give a wrong reading of what spell was cast, so if the appearance is plainly an effect of the casting, successful Spellcraft is proof that it's fake as far as the Spellcraft user is concerned.
If the apparent event is such that it could have really happened at the same time the wizard cast the illusion, the Spellcraft user has been informed it's an illusion, but doesn't have proof, so he must study it carefully or interact with it to get a save at +4. For example, if two real orcs charge into the room at the same time the illusionist creates images of four orcs, the Spellcraft user gets +4 to his saves, but can't automatically tell which orcs are false.