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Jeff:

Generally, the NPC/Monsters will know what to do if struck by a Killing Strike. You, as a character, simply has to try and see what happens. As has been said before, the KS hit is often considered a Mortal strike, 4 points, (if made to the Torso) and normal damage (if made to anywhere else). However, different NPCs/Monsters may have different kinds of defenses.

To know whether a monster is KSable, you'd need either plain experience encountering the monster (e.g. trial and error, learning from past fights) or Animal Lore.

Monsters that take less damage, do not seem to be overly hurt by a KS, or are immune will call out what kind of damage was done. Or, at an opportune time, ask the NPC what happened.