Detecting a Trap. Traps can be detected with:
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an active search;
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certain spells;
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in passing.
A trap’s description specifies the checks and DCs needed to detect it, disable it, or both.
Magic Traps. A magic trap is usually more difficult to detect than a non-magic trap.
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Any character can attempt an Intelligence (Arcana) check to detect a magic trap, in addition to any other checks noted in the trap’s description. The DCs are the same regardless of the check used.
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Some (but not all) magical device traps can be detected with a Wisdom (Perception) check. Go here for examples.
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Spell traps usually require an Intelligence (Investigation) check to be found. Go here for examples.
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Use the find traps spell to determine that a trap is present or the detect magic spell to detect the magical aura surrounding a magic trap.
Proficiency with thieves' tools. See XGtE p. 84 for how proficiency with such tools effects your Wisdom (Perception) checks and Intelligence (Investigation) checks when looking for traps.
Non-Magic Traps. Usually, some element of a trap is visible to careful inspection.
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The most common way to detect a trap is by actively seeking evidence of it; you make a Wisdom (Perception) check against the trap’s DC.
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Less frequently the GM may ask you to make an Intelligence (Investigation) check instead of, or in addition to, a Wisdom (Perception) check.
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How well you can see in an area, which might be lightly or heavily obscured, effects your chance of success.
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If a tool applies to either type of check and you are proficient in both, the GM could grant you advantage on the check.
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Even if there is no active search, the GM can compare the DC to detect a trap with each character’s passive Wisdom (Perception) score to determine whether anyone notices the trap in passing.
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Use the find traps spell to determine if a trap is present (but not its specific location).