Exactly.

And the real problem is the way humans decide sentience in the first place - by how much the machinery acts as if it were sentient. There is no other information - just our perception. If we imagine two agents, A and B, one sentient and one insentient, but both acting identically, we theoretically couldn't decide which one is which.

So whether or not there is sentience in a machine then becomes a question as unanswerable as whether there is something that exists outside the universe we can perceive. We cannot know what we cannot possibly perceive.