I really should read things more carefully. "Regardless of how he interprets it". DOH. Would you please be kind enough to give me an example of what you mean? I have a feeling I know what you're talking about but it's St Patrick's Day and Guinness is good for you after all
Suffice to say I'll answer while recovering from the hangover.
i.e. Someone submerses his hand in boiling water, then pulls it out quickly. You can be reasonably sure, barring some strange neurological disorders, that he felt pain, because he pulled his hand out. That was the external criteria you were looking for, to verify that he felt pain, regardless of whether pain for him feels way different than it feels for you. You never explained pain to him, but now he knows what it feels like, to him, and can build off of it. So you're basically reducing things down to the point where someone can't possibly be screwing up the subjective/"internal" aspects, so long as the objective/"external" are present.
It's nothing terribly profound, just I found it interesting how much the model of transmission was being emphasized and taught as much as the content.