I know I’m conscious. I only extend the assumption to you that you’re conscious because I’m hoping you will extend the same to me. But I have no way of knowing that you are. And you (if you really are conscious) have no way of knowing that I am.
This is a good starting point, but it doesn't have to be the _conclusion_ to this line of thinking. Philosophically: You can being building criteria for consciousness; the things you look at in yourself that tell you are, and then begin looking for that (or symptoms of that) in other people. Anecdotally: you can totes spot "unconscious" people. You can even watch people gain consciousness, if you watch 'em in the rig…
This is only true for the outward behavior we define as consciousness. The experiential part of it (qualia and such) can not be described in objective terms (try describing 'redness' by itself). That is the hard problem of consciousness.
What you can do in that realm is experiment with n=1 using optical illusions, psychedelics and dissociatives (in ascending order of how weird you want things to get).