It is not the duty of a person with a disability to justify themselves to every skeptic they encounter. That is both unrealistic and discriminatory, especially as we would endlessly encounter people who would not be convinced no matter what we say. Disability accomodation laws have made this crystal clear for decades.

Even people with handicap permits regularly get questioned as to if they are "genuinely" disabled, and the only thing that tamps it down at all is the fact that an establishments that do so are financially discouraged from it.

Someone faking a disability is a nuisance, someone discriminating over a disability is a human rights violation.