Well, not sure if this is technically a rules question since I can't seem to dig up much information on it, but...
How much information can you actually get from residual magic auras?
Examples:
1.
You hear the murmuring of a spell in the next room, which you identify as someone casting teleport and leaving. You enter the room, see nothing, and want to find out who it was teleporting and where they were going. Maybe even if they took anyone or anything with them.
As far as I'm aware, if you want to actually find out this sort of thing, rules as written you've got no chance.
2.
Someone casts sending, with you as a recipient. You've never met them, and they don't tell you their name or location. They might even just spam you with nonsensical garbage just to annoy you, sort of like a magical telemarketer.
Again, as far as I'm aware, there's just nothing you can do in this situation to find and smite your pesky prankster (since you know nothing about them). While you can send a message back as the spell says, for some reason that's all you can do.
3.
Your best pal is dead, killed by a madman who teleported in from another planet, used a maximised timestop (from a rod, probably) and lots of delayed blast fireballs (also meta'd up to this lunatic's standards) before teleporting home. You don't find out for a day or two, but you head out to investigate.
Yup, there's this big ol' aura that basically tells you what happened. It doesn't help you at all with finding your friend's killer, or even help you with recognising any more of the killer's magic if you found it again.
So, in case this isn't clear enough, this comes in a couple of parts.
1 - Can you tell who cast a spell?
2 - Is it possible to tell where the spell was cast?
3 - Are you able to find out specific details about a spell (i.e. what monster did they summon, where did they teleport to, what was it an illusion of, &c.)
4 - In the case of short-lived auras such as from level 1 spells, can you at all enhance their strength/still detect them some time afterwards?
5 - Can you tell exactly how long ago a spell was cast (especially useful when the aura lingers for days)
And continuing on to slightly more advanced questions
6 - If you don't know the caster, could you recognise their magic if you saw it again?
7 - Could you locate the sites of other spells the caster's cast?
8 - Can you use the residual magic to recast the spell (to, say, temporarily speak to a summoned creature)?
I'm probably being dumb because this feels like it ought to be fairly basic stuff, but I can't see any sort of reference to it.
The Detect Magic spell has info on Lingering Auras:
Lingering Aura: A magical aura lingers after its original source dissipates (in the case of a spell) or is destroyed (in the case of a magic item). If detect magic is cast and directed at such a location, the spell indicates an aura strength of dim (even weaker than a faint aura). How long the aura lingers at this dim level depends on its original power:
.
Original Strength...Duration of Lingering Aura
Faint....................1d6 rounds
Moderate..............1d6 minutes
Strong.................1d6x10 minutes
Overwhelming.......1d6 days
So for that length of time, you can detect the lingering aura of magic that was used but is no longer present.
Once you know there is a lingering aura, you can concentrate to find out the following:
1st Round: Presence or absence of magical auras.
2nd Round: Number of different magical auras and the power of the most potent aura.
3rd Round: The strength and location of each aura. If the items or creatures bearing the auras are in line of sight, you can make Knowledge (arcana) skill checks to determine the school of magic involved in each. (Make one check per aura: DC 15 + spell level, or 15 + 1/2 caster level for a nonspell effect.) If the aura emanates from a magic item, you can attempt to identify its properties (see Spellcraft).
But nothing else. Knowing the names or head counts of a teleport spell is not possible. Knowing the name or location of the telemarketer with a Sending spell is not possible. Recognizing unique caster signatures so you could know that caster's magic in the future is not possible.
1 - Can you tell who cast a spell? Not if they're not present, but if you look into a room with a dozen people and one of them has cast a spell, you could tell which one. But if he's left that room and you're detecting the lingering aura, you'll never know it was that guy who cast it.
2 - Is it possible to tell where the spell was cast? Yes, in the third round.
3 - Are you able to find out specific details about a spell (i.e. what monster did they summon, where did they teleport to, what was it an illusion of, &c.) No. Just the stuff I quoted here.
4 - In the case of short-lived auras such as from level 1 spells, can you at all enhance their strength/still detect them some time afterwards? Only for 1d6 rounds.
5 - Can you tell exactly how long ago a spell was cast (especially useful when the aura lingers for days) Apparently not.
6 - If you don't know the caster, could you recognise their magic if you saw it again? No.
7 - Could you locate the sites of other spells the caster's cast? No.
8 - Can you use the residual magic to recast the spell (to, say, temporarily speak to a summoned creature)? Not even close. There are no rules for anything like this.
Many of the things you are asking seem like good ideas and they're recurring themes in much of fantasy fiction, but Pathfinder does not have rules that allow this.
They would great make house rules though.
Well, at least I wasn't being totally dumb (2 I should have known better. Duh.) and there really are no rules for it. I was hoping there'd be a section in Magic or Spellcraft or something that I'd just overlooked that would detail that.
Feels like these ought to be in the rules though. Maybe Ultimate Intrigue, when it comes out, will have this sort of thing. I've got an idea how I'd do it as a houserule for the time being at least.
Thanks for that support!
EDIT: Actually, detect magic doesn't indicate if the aura of a spell emanates from the location it was cast or the location it took effect, having thought about it. There's a significant difference between the aura of a fireball if it's where it exploded or if it's from a wizard flying 100 feet in the air. It doesn't even indicate if it should be both.
Also, another question which will also have the answer no is "Can you tell if a spell came from an actual caster or a scroll/wand/potion?".
Well, at least I wasn't being totally dumb (2 I should have known better. Duh.) and there really are no rules for it. I was hoping there'd be a section in Magic or Spellcraft or something that I'd just overlooked that would detail that.
Feels like these ought to be in the rules though. Maybe Ultimate Intrigue, when it comes out, will have this sort of thing. I've got an idea how I'd do it as a houserule for the time being at least.
Thanks for that support!
EDIT: Actually, detect magic doesn't indicate if the aura of a spell emanates from the location it was cast or the location it took effect, having thought about it. There's a significant difference between the aura of a fireball if it's where it exploded or if it's from a wizard flying 100 feet in the air. It doesn't even indicate if it should be both.
Also, another question which will also have the answer no is "Can you tell if a spell came from an actual caster or a scroll/wand/potion?".
There is the Witch hunter archetype that does soemthing like what you are describing.
http://www.d20pfsrd.com/classes/base-classes/inquisitor/archetypes/paizo--- inquisitor-archetypes/witch-hunter
That's a cool archetype, and I suppose you could interpret that as being able to follow lingering auras, in which case it would work well with a one level dip into sleepless detective.
It would be unfortunate to have to steal their thunder, but I still think that rules to cover my questions would be good. To me, it sounds stupid that a master of magic can't track down an annoying telemarketer by themselves; their only option is to contact another plane in the hope that someone elsewhere knew (and they would also inexplicably know despite not being able to do the same thing).