You can tell someone’s true intentions and feelings by paying attention to things like body language, inflection, and your own intuition.
A successful Insight check allows you to resist the effects of some interaction skills, becoming aware of the other person’s true intent. You can also use the skill to tell when someone is behaving oddly or for assessing trustworthiness.
The GM makes a secret Insight check to determine if your hero senses the true nature of an illusion (DC 10 + Illusion rank). Success means you pick up on a flaw in the illusion, sensing it is not real.
You can make a Insight check to notice someone acting under outside influence. The DC is 10 + the rank of the effect or skill affecting the subject. If you succeed, you notice the subject is not acting entirely of his or her own will. Three or more degrees give you a general idea of what is influencing them (and perhaps even whom, depending on the situation and the Gamemaster’s judgment).
With an Insight check, opposed by Deception, you can tell if someone is trustworthy and honorable (or not) upon meeting them. You can also make an Insight check (DC 20) to evaluate a social situation, getting a feel for the overall mood and prevailing attitudes. Two or more degrees of failure on either check mean you misinterpret the signs, so the GM may make these checks for you in secret.
You can use Insight to pick up on hidden messages sent via the Deception skill.
Make an Insight check when called to do so to resist or overcome the effects of certain interaction skills, such as Deception or Intimidation. If the result of your check exceeds your opponent’s, you are unaffected by their attempt to influence you.
D20Hero SRD Skills article- http://www.d20herosrd.com/4-skills