Mystic Bloodhound

 
Prerequisite(s): Caster level 3+, Spell Focus (Divination)
 
Benefit: You have studied the unique patterns present in the signatures of magic spells, allowing you to gain insight into the spellcasters abilities. When you are affected by a spell, you can choose to automatically fail the saving throw to gain insight on the caster. You gain a +2 to all saves against future spells cast by that caster and can tell his general location after he casts a spell. This ability can be linked to up to three spellcasters at a time.
 
Source: Mongoose Ultimate Feats