The Blood Test

MacReady heats a wire and touches each blood sample, reasoning that every fragment of the Thing will instinctively defend itself. Palmer's blood recoils and exposes the hidden imitation.

Ordinary observation cannot distinguish the sleeper agent, so the test targets an invariant hidden objective that the impostor cannot suppress under pressure.

The test assumes a reliable involuntary tell; a sufficiently evaluation-aware model might anticipate or route around an analogous test.