I give you, from my campaign, a PrC based off detectiving in a highly-magical city. It's designed to be for the city watch in Sigil, the super-high-magic city in Planescape. It would probably also be useful in other high-magic metropolises, like Waterdeep. It gives abilities useful when your subjects have Spell Resistance and Teleport, and spells that would be useful for a detective. They can Speak with Dead to a corpse, and See Invisibility if the perp is a Wizard. That kind of thing.
Code:
Class Base Spells per day
Level Attack F R W Special
Bonus 1st 2nd 3rd
1 +0 +2 +0 +0 Sense Invisibility 1 - -
2 +1 +3 +0 +0 Skill Focus, Spell Penetration 1 - -
3 +2 +3 +1 +1 Track Teleport 2 1 -
4 +3 +4 +1 +1 Skill Focus, Greater Spell Penetration 2 1 -
5 +3 +4 +1 +1 Follow Teleport 3 2 1
Requirements
To qualify as an Planar Detective you must meet the following requirements
Feats: Seasoned Officer or Subdue Criminal (special feats from my campaign. You could substitute Alertness instead, maybe).
Sills: Gather Information 8+ ranks, Intimidate 4+ ranks, Search 4+ ranks
Other: BAB 4+
Class Skills
A Planar Detective class skills (and the key ability for each skill) are Appraise (Int), Bluff (Cha), Climb (Str), Concentration (Con), Craft (Int), Disguise (Cha), Diplomacy (Cha), Gather Information (Cha), Hide (Dex), Innuendo (Cha), Intimidate (Cha), Jump (Str), Knowledge (all), Listen (Wis), Perform (Cha) Profession (Wis), Search (Int), Sense Motive (Cha), Spellcraft (Int), Spot (Wis), Wilderness Lore (Wis).
Skill Points at Each Level 6 + Int modifier.
Class Features
All of the following are class features of the Planar Detective
Hit Die: d8
Weapon and Armor Proficiency: Planar Detective gain proficiency with the sap and mancatcher, if they did not already have them. They gain no other new armor or weapon proficiencies.
Sense Invisibility: Whenever there is an invisible creature or object within the Planar Detective's line of sight, he may make a Spot check with a DC equal to 10 plus the invisible creature's level/hit dice. If he succeeds, he knows there is an invisible creature nearby, but not it's exact location.
Spell Penetration: At 2nd level, the Planar Detective gains the Spell Penetration feat for free.
Skill Focus: Upon reaching 2nd level, and again at 4th level, the Planar Detective gains the Skill Focus feat for free. He may only apply it to one of the following skills: Bluff, Diplomacy, Gather Information, Innuendo, Intimidate, Listen, Search, Sense Motive, or Spot.
Track Teleport: A Planar Detective may sense the location another teleported to. To do so, the Planar Detective must spend a full round standing approximately where the teleporter teleported from and concentrate. The Planar Detective then gains a vision of where the subject teleported to. This power works with all forms of instantaneous travel, including Teleport Without Error and Planeshift
Follow Teleport: At 10th level, the Planar Detective gains the ability to follow a suspect that has teleported away. Whenever a creature teleports, the Planar Detective may choose to be automatically teleport along with the caster so long as the Planar Detective is within 5' of the teleporter. This power works with all forms of instantaneous travel, including Teleport Without Error and Planeshift
Spells per day/ Spells known: beginning at 1st level, a Planar Detective may cast spells as shown on the above table. These are arcane spells and the Planar Detective prepares and casts them as a wizard, though they know all the spells on the spell list and suffer no arcane spell failure for wearing armor. The Planar Detective's spell list is as follows:
1st - Detect Thoughts, Locate Object, See Invisibility, Comprehend Languages, Command, Detect Magic, Read Magic
2nd - Speak With Dead, Locate Creature, Hold Person, Find Traps, Tongues
3rd - Hold Monster, Dimensional Anchor, Invisibility Purge, Dispel Magic, Dictate
You can find it on the web here:
http://www.zachshuford.com/Kriegstanz/harmonium.html