I don't know that I'm conscious. (And to avoid the inevitable snark, using "I" in written text doesn't demonstrate I do.)

If I wanted to know that something is, say, a triangle, you could tell me what it means for something to be a triangle. I could then check things and say "yup, that one is a triangle".

If I were to instead be puzzled about "red", you couldn't describe being red to me, but you could at least show me examples of things that are all red. I could verify that the perceptions I have of those things are similar. Furthermore, I could then point to other objects, say "these produce the same perceptions in me as the first objects", and discover that you agree with me on those objects producing the same sensation, even if I can't directly compare the sensation in my head to the sensation in yours.

But even this isn't possible for "consciousness". Nobody can perceive more than one example of consciousness. Even assuming that I am conscious and can perceive that, there's no commonality--there aren't any sets of things which both of us will perceive as consciousness, that I can use to generalize from to figure out whether some unknown thing I can perceive is also consciousness. If I only ever saw one red object in my lifetime, and the only red object you ever saw in your lifetime was a different object, how could either of us know that the two are the same color?