Search. You might want to search for a hidden object, a secret door, or a hidden foe.
Depending on the nature of your search, the GM might have you make a:
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Intelligence (Investigation) check.
See here for difference on the two.
During a combat round, your effort may require the Search action. Or not.
Is your search a success?
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If you search for an object, your check total is compared to the DC as determined by the GM. See details on detecting a trap.
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If you try to discover a hiding enemy, your Wisdom (Perception) check is contested by the enemy's Dexterity (Stealth) check. Go here for details on hiding.
Whether an area is lightly or heavily obscured is an important factor in the success of a search.