Thomas Nagel produced a famous philosophical thought experiment "What Is It Like to Be A Bat?" In it, he argued that the reductionist understanding of consciousness was insufficient, since there exists beings - bats - that have conscious experiences that humans cannot understand. We cannot know what "it is like to be a bat", and looking reductively at bat brains, bat neurones, or the laws of physics, cannot (allegedly) grant us any understanding of this subjective experience. Therefore there remains an unavoidable subjective component to the problem of consciousness.
I won't address this issue directly (see for instance this, on the closely related subject of qualia), but instead look at the question: suppose someone told us that they actually knew what it was like to be a bat (as well as what it was like to be a human). Call such a being a vampire, for obvious reasons. So if someone claimed they were a vampire, how would we test this?
We can't simply ask them to describe what it's like to be a bat - it's perfectly possible they know what it's like to be a bat, but cannot describe it in human terms (just as we often fail to describe certain types of experiences to those who haven't experienced them). Could we run a sort of Turing test - maybe implant the putative vampire's brain into a bat body, and see how bat-like it behaved? But, as Nagel pointed out, this could be a test of whether they know how to behave like a bat behaves, not whether they know what it's like to be a bat.
I posit that one possible solution is to use the approach laid out in my post "the flawed Turing test". We need to pay attention as to how the "vampire" got their knowledge. If the vampire is a renown expert on bat behaviour and social interactions, who is also interested in sonar and paragliding - then them functioning as a bat is weak evidence as to them actually knowing what it is like to be a bat. But suppose instead that their knowledge comes from another source - maybe the vampire is a renown brain expert, who has grappled with philosophy of mind and spent many years examining the functioning of bat brains. But, crucially, they have never seen a full living bat in the wild or in the lab, they've never watched a natural documentary on bats, they've never even seen a photo of a bat. In that case, if they behave correctly when transplanted into a bat body, then it's strong evidence of them actually understanding what it's like to be a bat.
Similarly, maybe they got their knowledge after a long conversation with another "vampire". We have the recording of the conversation, and it's all about mental states, imagery, emotional descriptions and visualisation exercises - but not about physical descriptions or bat behaviour. In that case, as above, if they can function successfully as a bat, this is evidence of them really "getting it".
In summary, we can say "that person likely knows what it is like to be a bat" if "knowing what it's like to be a bat" is the most likely explanation for what we see. If they behave exactly like a bat when in a bat body, and we know they have no prior experience that teaches them how to behave like a bat (but a lot about the bat's mental states), then we can conclude that it's likely that they genuinely know what it's like to be a bat, and are implementing this knowledge, rather than imitating behaviour.