It's a reasonable inference given your similar biology and behavior. Doubting your subjectivity is on the level of doubting that anything exists outside my experience, or doubting that the universe existed more than five minutes ago. I can't prove those things are true, but I have no good reason to have such doubts.

Still, it remains a philosophical problem, even more so for animals or robots like Data. That's what Ned Block called The Harder Problem of Consciousness. But here I think we just have to accept that our knowledge of others and the world lacks certainty. We trust our senses and inferences to form a reasonable view of the world, but we can never be sure.