Relevant Ability: Wisdom
Your senses allow you to notice fine details and alert you to danger. Perception covers all five senses, including sight, hearing, touch, taste, and smell.
Check: Perception has a number of uses, the most common of which is an opposed check versus an opponent’s Stealth check to notice the opponent and avoid being surprised. If you are successful, you notice the opponent and can react accordingly. If you fail, your opponent can take a variety of actions, including sneaking past you and attacking you. Perception is also used to notice fine details in the environment. The DC to notice such details varies depending upon distance, the environment, and how noticeable the detail is.
Action: No if someone is attempting to sneak past you, Move equivalent if you are actively watching for someone and full round if you are searching for clues.