Spell Resistance, Successful Saves and Immunity.

[Ignominia]

Question: if a PC spell caster hits an enemy with a spell, and they either: Make the save, succeed with spell resistance, or are flat out immune, can the PC tell?

Example: a huge monsterous scorpion (vermin) is immune to mind effecting spells. If someone casts Doom (a mind effecting spell) do they know that it's had no effect?

If an enemy has a way to avoid the spell from multiple sources (say SR and saves) and is successful with ONE of those sources, do the PCs know which?

And flip side to the same question, do PCs Know when they have succeeded?

[Deophaun]

Yes. In general, you know who was affected and who was not.

Once you know which creatures (or objects or areas) are affected, and whether those creatures have made successful saving throws (if any were allowed), you can apply whatever results a spell entails.
Now, it should be noted that the "you" in this context has been used to refer to the individual casting the spell throughout the section, e.g.

To make a caster level check, roll 1d20 and add your caster level...

If you ever try to cast a spell in conditions...
So the caster knowing who is affected and who is not is an integral part of resolving a spell.

This rule is also implied by the existence of things such as the False Pretenses feat, which lets you deceive casters into thinking that their charm/dominate spells worked on you even if you successfully save.

[TuggyNE]

Yes. In general, you know who was affected and who was not.

Only for targeted spells (http://www.d20srd.org/srd/magicOverview/spellDescriptions.htm#succeedingonaSavingThrow); area spells give you no such feedback.
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. Likewise, if a creature’s saving throw succeeds against a targeted spell you sense that the spell has failed. You do not sense when creatures succeed on saves against effect and area spells.

It's not clear whether SR has the same semantics, and even less clear whether immunity does.