Detect a Hidden Foe
Use your sight, hearing, or even sense of smell to detect the presence of lurking enemies. See examples.
If the outcome is uncertain, the GM will usually call for a Wisdom (Perception) check. (77, 82) Make sure you describe your effort!
If a perception check is called for, remember that:
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Poor lighting, fog, and other obstructions may give you disadvantage.
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Moving at a fast pace can give you disadvantage.
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Natural obstructions may limit how far you can see.
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There are limits on how far you can hear.
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See DMG 244 for Tracking DCs.
Using magic. Sometimes you can use spells or magic items to help you discover a hidden foe.
Passive Perception. Even if you are not actively searching, the GM can determine whether you detect the presence of a hidden enemy in passing through use of a passive Wisdom (Perception) check. See the example.