Search/Detect
You can actively search for a secret door, a concealed trap, a hidden foe, or a hidden object.
If you are using your senses (sight, hearing, touch, smell, or taste), then the GM might have you make a Wisdom (Perception) check.
Depending on the nature of your search, the GM might have you make a Wisdom (Perception) check or an Intelligence (Investigation) check. (94)
You may also be able to detect something hidden in passing, that is, even when you are not actively searching for it. You can also use magic to find things.
Active Search. Depending on the nature of your search, the GM might have you make a Wisdom (Perception) check or an Intelligence (Investigation) check. (94) Any character can attempt an Intelligence (Arcana) check to detect a magic trap, in addition to any other check noted in the trap's description. (195)
GM Companion note: You may also be able to detect a hidden foe, a secret door, a concealed trap, or some other thing, in passing (when not actively searching). The GM will secretly determine whether you succeed with a passive Wisdom (Perception) check. (See 78, 81, 195)
See more on detecting a hidden foe.
See more on a listen check.
Describe your search in detail!
Is a check even necessary?
Make a "quick glance" during combat.
GM discretion with passive detection.