Typically used with: Cunning.
In short: sense it. Characters with ranks in Perception are cognitive masters, picking up the sounds, smells, and sights that the rest of us don't or can't. This skill can be used to see or hear stealthy characters, witness your change purse being snatched away, eavesdrop on conversations, overhear the faint sounds around you, notice minute details of an object, or realize you forgot deodorant.
If you gain the distracted condition, you take a −4 penalty to Perception checks as your attention is fixated on something other than your surroundings. You also take a penalty to your Perception check if you're surrounded by impediments to your senses. Moderately poor conditions (like a patch of fog or a noisy marketplace) impose a −4 penalty. Extremely poor conditions (like inky darkness or a riot) impose a −8 penalty.
The GM might want to inform those who roll successful checks what their characters perceived in private or with notes. Doing this helps any players who didn't meet the DL role-play truthfully, as they weren't privy to the successful result.
You can make a Perception check to pick up on anything that can stimulate your senses. The higher the DL, the less noticeable the stimuli (e.g. further away, fainter, shorter in duration, more obstacles in between). Perception checks can involve any of the five physical senses.
The DL to perceive stimuli increases by 1 for every 10 feet of distance away from you. You can't perceive sights and smells separated by solid walls, but you can definitely perceive sounds that way. The DL increases by 4 if it's on the other side of a wooden wall or door, and the DL increases by 8 if it's on the other side of a stone or metal wall or door.
One Perception check takes no time at all, however a second attempt, trying to see or hear something you did not previously, takes 3 AP. If time isn't an issue, such as when the thing to perceive is stationary or constant, you can Take the Best on this check.
You must make a Perception check to notice the opponent creeping up behind you or the monster hiding under your bed. If your Perception check meets or exceeds the opposing Stealth check, you notice them. If the Stealth check is higher, you have no idea they're nearby, and gain the wide-eyed condition against the first attack from the sneaky fellow.
You must subtract your Size from this roll, as it's easier to notice a larger creature and harder to notice a smaller one. For example, a firna is hiding from a juren. The firna gets +1 to his Stealth check, while the juren takes −5 to his Perception.
When someone tries to take an item from your person without your knowledge, you get to make a Perception check to notice them before they nab your stuff. If you meet or exceed the opposing Thievery check, you spot the pickpocket and get a chance to hold onto your object. What happens next is up to you, but if the thief wants the item badly enough, combat begins and they'll have to try prying it from your person.