Discern why a spell failed

[noce]

Suppose I cast Charm Person on a creature, and nothing happens.

It can be for any of these reasons:

target passed Will save
target has SR I didn't beat
target is under a Mind Blank spell
target has immunity to charm effects
target has immunity to mind affecting effects
target is not humanoid
another reason


How can I tell for which reason the spell failed?
For example, is SR blocking a spell any different from a passed save?
After the spell fails, can I detect the presence of an ongoing Mind Blank spell using Spellcraft?

[DarkWhisper]

Suppose I cast Charm Person on a creature, and nothing happens.
How can I tell for which reason the spell failed?


target passed Will save

Likewise, if a creature’s saving throw succeeds against a targeted spell you sense that the spell has failed.


target is not humanoid

Spot and/or appropriate Knowledge check.


target has immunity to charm effects
target has immunity to mind affecting effects


Appropriate Knowledge check.


target has SR I didn't beat

Spell resistance prevents a spell or a spell-like ability from affecting or harming the resistant creature

Since a caster knows whether a target saves versus a (targeted) spell, the target not being affected by a targeted spell should also be evident.
Thematically (i.e. not RAW), SR opposes a caster's spell, forcing the caster to 'work' for his spell affecting the target (trying to overcome the SR); this should be noticeable.


For example, is SR blocking a spell any different from a passed save?
Successfully saving versus a (targeted) spells is specifically spelt out in the rules as being known by the spell's caster.



After the spell fails, can I detect the presence of an ongoing Mind Blank spell using Spellcraft?

20 + spell level: Identify a spell that’s already in place and in effect. You must be able to see or detect the effects of the spell.
Having a spell negated by Mind Blank might be sufficient for a spellcraft check. Alternatively, Detect Magic or similar should allow such a spellcraft check.

[Telonius]

Immunity to charms and compulsions can come from a variety of sources. If you can see that the creature isn't humanoid, you could make a Knowledge check. But if it's (for example) a Doppelganger (aberration) under the appearance of a human, unless you somehow independently know that it's a Doppelganger, a Knowledge check isn't going to help you.

[Khedrac]

Suppose I cast Charm Person on a creature, and nothing happens.

It can be for any of these reasons:

target passed Will save - all you learn here is "spell failed" so this result is process of elimination.
target has SR I didn't beat - You know if you got to make a caster-level check, and I think (not sure) you know if it succeeds or not.
target is under a Mind Blank spell - DC28 spellcraft check to identify a spell in place from observing its effects - blocking your spell is definitely its effect..
target has immunity to charm effects - all you learn here is "spell failed" so unless there is a knowledge check appropriate (as in it is a racial immunity) you are out of luck here.
target has immunity to mind affecting effects - as charm immunity
target is not humanoid - not sure if you know the fdifference between spell failed because of save and spell failed because of invalid target. Again knowledge check is your friend (and know.local should be able to tell you that it is not a humanoid).
another reason - - if it is because of an existing spell effect then you get a spellcraft check DC20 + spell level, if you spell is countered you may get a spellcraft check to spot the counter.


There is also:
Spellcraft DC 30 or higher - Understand a strange or unique magical effect, such as the effects of a magic stream. Time required varies. No retry.

Note on Spellcraft. For a spell being cast the SRD states that you must be able to see/here the spells somatic/verbal compoinents, which implies spells without cannot be identified this way. If your DM has Tome and Blood (the 3.0 book) then it is just +2 to the DC for each component you cannot observe so you can still attempt the check. Your DM may vary.

You should definitely be able to attempt the spellcraft check for an ongoing spell from observingits effects if it just blocked your spell, a DM who says "no" to that is panicing to stop you doing something unexpected.

[Zarrgon]

target passed Will save
target has SR I didn't beat

In both cases the caster knows this: either the target used it's will power to shake off the spell or it has resistance to magic.



target is under a Mind Blank spell

This goes for all spells: if you cast a spell and it fails that is ''detecting the effect of the spell", so you can roll a spellcraft roll.



target has immunity to charm effects
target has immunity to mind affecting effects

And add *Target has a magic item that does one of the effects and *target is immune to magic.

This one you don't have much of a chance of figuring out. Spellcraft is no help. A knowledge check can tell you what a creature is immune too.

However there are a lot of ways to become immune, such as class abilities, templates(the ones that are not visually easy to spot), and most of all magic items.

There is nothing much to do here, other then stop and do a mini game, that is do nothing else, to discover where the immunity comes from through game play.



target is not humanoid

In most cases you should be able to tell this by looking, but it's an easy knowledge roll.



another reason

A twist here is illusions and changing shapes and even a disguise. A crafty elf could disguise themselves as a human and so forth so you can't see what they really are and know about them. Unless you first get through that.


On a basic level, you should not worry too much about a spell not working: it happens. For the most part, an a balanced game, most ''important'' targets will have defenses. Even more so vs simple low level magic. You can just accept that and move on...and keep in mind it's mostly only the important targets: minions, servants, and such likely are easy targets.