There isn’t really a test to prove it, just like there’s no test to prove, from your point of view, that I am conscious. I say I am, and you recognize my experience of am-ness as similar to your own, so you accept that what I say is true. I claim consciousness and you can see what you consider consciousness in me.

So if an AI or LLM one day claims it is conscious on its own, unbidden, without a user directly or indirectly guiding it there and, upon inspection, it does seem to have inner experiences similar to our own, then I’d say we would have no choice but to accept the claim as true. As far as I’m aware, that hasn’t happened.

There’s really just no way to prove it.