> But how do you know for sure?

Presumably by intelligent, competent, analytical direct consideration.

The non-layman should get a pretty good idea at assessment (in this case, "where is nociception and is it there" etc.)

Allow me a relatively distant reference, for fun. Prosecution: "How do you know the patient was dead at the time?" // Doctor: "Because his brains were sitting on my table" // "And can you know for sure he was dead?" // "He could have been alive, brainless and practicing law maybe".

Which wants to mean, at least: surely there are subtle non easy questions, but still there exist thresholds of competence enabling better assessment. (And subtle doubts call for subtle defences, to be non-gratuitous.)