How do you know when desensitization training is complete and the horse is genuinely ready for real-world exposure?

Knowing when desensitization training is complete — when a horse is genuinely ready for the real-world situation rather than just performing adequately in a controlled training environment — is a judgment call that Clinton Anderson, Pat Parelli, and Warwick Schiller all address, and their criteria are more specific than simply that the horse stood quietly. Anderson's standard is that desensitization is complete when the horse shows no response to the stimulus under any conditions the training environment can simulate — when it yawns, licks and chews, or drops its head while the stimulus is active. Standing quietly is not enough because standing quietly can indicate shut-down as much as genuine confidence. The horse that is genuinely desensitized shows active processing signs even while the stimulus is ongoing, not just when the stimulus stops. He also tests generalization: a horse desensitized to a tarp in the round pen is tested with the tarp in a different location, from a different angle, in a different wind condition, while other horses are present, and during a session when the horse is already slightly fresh or stimulated. If the response remains consistent across these variations, the desensitization has generalized enough to be reliable. Warwick Schiller adds the nervous system baseline test: is the horse's overall baseline nervous system state calm enough that the desensitized stimulus does not push it past its threshold even when the horse arrives at a session already slightly activated? A horse that handles the tarp perfectly when it is calm but spooks at it when it has been stabled for a week has context-specific desensitization rather than genuine habituation. Parelli frames readiness as the horse showing curiosity rather than concern — approaching the formerly frightening stimulus voluntarily, investigating it, and treating it as interesting rather than threatening. This shift from avoidance to investigation is the clearest possible indicator that the horse has genuinely updated its assessment of the stimulus from dangerous to safe.