We can measure the physiological activity, but not whether it gives rise to the same sensations that we experience ourselves. We can reasonably project and guess that they are the same, but we can not know.
In practical terms it does not matter - it is reasonable for us to act as if others do experience the same we do. But if we are to talk about the nature of conscience and sentience it does matter that the only basis we have for knowing about other sentient beings is their self-reported experience.
