You gain Heroic Inspiration when you do any of the following.
Damage Vulnerability
Deal a type of damage to which a creature has Vulnerability.
Find Weakness
Trigger a creature's specific weakness, such as Shadow's Sunlight Weakness or dealing damage that prevents a creature's Regeneration.
Guess the Monster
Guess a creature's specific kind (such as a mimic or lich) before seeing it. The GM confirms if you are correct when the creature is revealed. You can't guess the identity of Humanoids.
Secret Doors
Discover a secret door.
Trapfinder
Detect or disarm a trap.
Treasure Hoard
Find an Uncommon, Rare, Very Rare, or Legendary magic item or treasure worth 100 GP+.