Suppose a cleric casts **Zone of Truth** on a captured enemy. Unbeknownst to them, the enemy is a rakshasa. Zone of Truth says (PHB, p. 289):

> 
> You know whether each creature succeeds or fails on its saving throw.
> 
> 
> 

But (MM, p. 257):

> 
> The rakshasa is immune to spells of 6th level or lower unless it wishes to be affected.
> 
> 
> 

Assuming the rakshasa doesn't wish to be affected by the Zone of Truth spell, does the caster know this?

More generally, does a caster know when targets are unaffected by a spell they have cast, either because they have made saving throws, or because they are outright immune to the spell or its effects (e.g., casting **Ray of Sickness** on a shadow demon, which is immune to poison damage and the poisoned condition)?