> The only basis I have for assuming you are sentient according to that definition is trust in your self-reports

Because the other person is part of your same species so you project your own base capabilities onto them, because so far they shown to behave pretty similarly to how you behave. Which is the most reasonable thing to do.

Now, the day we have cyborgs that mimic also the bodies of a human a la Battlestar Galactica, we will have an interesting problem.