[ars]

How do I know another person is conscious? For all I know I'm the main character and you are all NPC's.

There's no actual way to know, we act that way because when we talk to others they have an internal state, and their programming is not alterable. There's also the argument that we act like everyone is conscious because of what would happen if we didn't.

And regarding that altered programming, there are drugs people can take that let them speak and seem intelligent without actually being conscious.

Specifically Ambien can make ordinary humans into basically AI's: despite seeming to be awake they are not actually conscious. And if you talk to someone on Ambien you can tell (if you are looking for it), so you can also tell if a computer is conscious.

https://www.nytimes.com/2011/12/04/magazine/can-ambien-wake-...