1. "A creature that successfully saves against a spell that has no obvious physical effects feels a hostile force or a tingle, but cannot deduce the exact nature of the attack." (Page 216 core rules)
2. Not unless there's something that gives it away.
Lets say you cast a spell (or use a SLA/Supernatural ability) on someone. He doesnt spot or hear you casting it (distance too far away, silent/still spell, etc). He succeeds at the spell save and there is no effect.
1.Does he know someone used a spell on him?
2.Is he able to identify the person who did it?
1) mkenner is correct
2) A target with Knowledge: Arcana can attempt a check with a DC equal to 25 + Spell Level to identify the spell that just targeted it.
However, this may not identify the source, unless the spell came from a spellcaster who was near enough thtat the target may have seen movement from a Somatic component or heard the Verbal components of the spell being cast.
Along this same line, does the *caster* know when someone makes their save? For some spells, it's not obvious. Zone of Truth, for example.
So, zone of truth is entirely worthless.
If you consider using forcing the target to make a will save to avoid telling you the truth to be useless, then yes Tvarog
Exactly. You cast ZoT, but you have no idea whether it worked on any particular target, so you still have no idea whether the target is lying. In which case, why bother to cast it? You just wasted a 2nd level spell slot for no benefit whatsoever.
By any reasonable standard, any character wise (or intelligent) enough to cast the spell would also be wise enough to not bother, simply because the outcome is still just as ambiguous as without the spell.
That is when you cast detect magic. You can see if there is a magical effect on the potential victim of ZoT.
Exactly. You cast ZoT, but you have no idea whether it worked on any particular target, so you still have no idea whether the target is lying. In which case, why bother to cast it? You just wasted a 2nd level spell slot for no benefit whatsoever.
By any reasonable standard, any character wise (or intelligent) enough to cast the spell would also be wise enough to not bother, simply because the outcome is still just as ambiguous as without the spell.
The people in the area of ZoT are more likely to tell the truth. It is useful.
Can you guarantee they tell the truth? Not really. But you could Persistent Spell it, for better odds.
Or, my favorite trick... Ask the person what spell effects they're currently under.
If they're under the effect of ZoT, they know they're under the effects of ZoT, so will say so. But if they saved, they know they saved against 'something' and would have to guess, if they were even trying to lie.
Works like a charm.