And yet, humans are assumed so without having to show it.
Suppose a computer programmed for scientific exploration came to earth that was only following a program, did not consider itself sentient or have a consciousness, but met humans who claimed they did, and they were then tasked with providing an argument that could convince this computer? How could they do so?
The computer would always argue that they are simply claiming to be due to evolution as it's advantages as it arouses sympathy, but that in reality they are soulless neural networks whose behavior simply evolved from selective pressure. They could never actually offer a compelling argument nor explain how the neural network inside of their cranium could ever produce self-awareness.