Bryk, perhaps if the PCs notice the enemy actually casting, I would allow them a "free" arcana check to see if they know what spell is being cast. If it is cast before the PCs enter the area and it has a visible or visceral effect, the I would give a passive check. But if it is already cast before the PCs enter the area, then unless there is a palpable effect that can be sensed by the PCs, I would not allow any chance to know.