Fnord wrote:
"I am not delusional, you are" is is something that a delusional person might say.
This. The only way to verify reality is via empirical methods really, to gain a sense of objectivity. Tangible records of events are key, such as medical records that show whether a person is capable of something. For example, if a person has no arms & another person claims that they punched them, you know that they are lying.