Use this skill to notice and pick up on things. Discerning details—such as clearly hearing conversation or reading fine text—requires at least three degrees of success on the Perception check.
In general, you have a –1 circumstance penalty to Perception checks for every 10 feet between you and what you are
trying to perceive. So hearing a noise from 50 feet away is a –5 modifier to your Perception check, for example.
The GM usually makes Perception checks secretly so you don’t know whether there was nothing to notice or you simply failed to notice it. The common sorts of Perception checks are:
Make a check against a DC based on how loud the noise is or against an opposed Stealth check. Normal conversation is DC 0, a soft noise DC 10. Listening through a door is +5 DC, +15 for a solid wall. While you’re asleep, hearing something well enough to wake up is +10 DC.
Make a check against a DC based on how visible the object is or against an opposed Stealth check. Something in plain sight is DC 0, while something subtle or easily over-looked may be DC 5, 10 or more. Visual perception is also used to detect someone in disguise or to notice a concealed object.
You can make Perception checks involving other sense types as well. Noticing something obvious to a sense is DC 0. Less obvious things are DC 10 or so, hidden things DC 20 or more, and discerning details requires at least three degrees of success, as usual.
You can make a Perception check every time you have the opportunity to notice something new. As a move action, you can attempt to notice something you failed (or believe you failed) to notice previously.
Various sensory effects provide modifiers on Perception checks.
D20Hero SRD Skills article- http://www.d20herosrd.com/4-skills