That's true. I was recently re-watching an old episode of Star Trek: The Next Generation, "The Measure of a Man", in which the android crewmember Data had to legally prove himself to be a sentient person.
It got me thinking, how exactly would a flesh-and-blood Human go about proving to others that they themselves were properly sentient? And if you can't do that for yourself, how could you prove sentience in other people, or even animals?
We take it on faith that other Humans are sentient, but how could we provide conclusive proof of that?