No conscious person can know if another person is conscious. There is no 'sensation' of experiencing another conscious. Given how many people can and have been fooled by AI, this lack of ability to sense another consciousness is clear.
It's a fair point, though. If one rejects a subjective experience without any doubt, the most plausible explanation is that they don't have that subjective experience.
If I tell you that I see numbers as colours, and you say "that's not possible", I would assume that you don't have synesthesia [1]. I wouldn't doubt the fact that I have it [2].
Similarly, if some people argue that consciousness is not something like I describe, I would naturally assume that they don't experience it the same way as I do.
