More specifically, how do you know whether they could or couldn't do the things you were asking them to do, especially if it was an invisible disability?
Doing it before isn't a good indicator.
Other people with the same disability doing it isn't a good indicator.
Not visibly trying isn't a good indicator.
Having a blowout (or not) isn't a good indicator.
Being too tired (or too wired) isn't a good indicator.