The Insight skill is used to discern intent and decipher body language during social interactions. Characters use the skill to comprehend motives, to read between the lines, to get a sense of moods and attitudes, and to determine how truthful someone is being. (Monsters rarely use Insight.)
    Insight is used to oppose Bluff checks and as the social counterpart to the Perception skill. The skill can also be used to gain clues, to figure out how well a social situation is going, and to determine if someone is under the influence of an outside force.
    When a creature uses Insight, it is making a best guess about another creature’s motives and truthfulness. Insight is not an exact science or a supernatural power; it represents the ability to get a sense of how a person is behaving.

        Action: Minor action. No action is required when opposing a Bluff check. The creature needs some amount of interaction with a target to interpret its words or behavior.
        DC: See the Insight table, except when countering a Bluff check, which is an opposed check.
        Success: The creature counters a Bluff check, gains a clue about a social situation, or senses an outside influence on someone.
        Failure: The creature can’t try again until circumstances change.

TaskInsight DC
Sense motives or attitudeModerate DC of the target's level
Sense outside influenceHard DC of the effect's level

        Read the mood of a crowd (easy DC)
        Discern who among a cagey group is the leader (moderate DC)
        Interpret enemies’ hand signs (hard DC)
        Recognize a creature as illusory (moderate DC of the effect’s level)

Update (6/19/2012)
Updated in Rules Compendium.

Published in Player's Handbook, page(s) 185, Rules Compendium, page(s) 147.