Quite literally the spell Detect Thoughts with the creature (which I believe you're referring to an Allip) in the detection cone, Dominate Monster targeted at the creature, or Telepathy trying to communicate with the creature or any other spell or effect that mimics those spells.
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I have misunderstanding in that cause undead are immune to mind-effectts...
Madness (Su) Anyone targeting an allip with a thought detection, mind control, or telepathic effect makes direct contact with its tortured mind and takes 1d4 points of Wisdom damage.
It doesn't care if the ability works on the undead, it cares about targeting it. AwesomenessDog is right, the only doubt is Detect thoughts, as it isn't a targeted spell or effect. I would have it activating the effect, but RAW it wouldn't.
Yeah, you could argue that it would or wouldn't before you've studied the Allip for 3 rounds and then do single it out to detect its specific thoughts, but if you try to read its mind specifically, you are targeting it for a save and therefore would take the damage.