You'd need to prove it's perfect. I can't help but think that would require the original mind itself to judge - no one really knows how an individual thinks outside of the individual.

I suppose if you assume the person to simply be outputs in response to inputs then you could probably create a near perfect simulation, but if you assume the mind has its own internal state that makes that mind distinct, and given that I observe myself as a thinking thing I hold that to be true, then it would take introspection to get closer to the truth.

How would you feel if someone created a simulation of you and then convinced your family and friends that you are no longer needed, and that your life could be terminated? Would you go willingly?

'How do you show he is lying?' maybe makes the scenario harder to digest than it needs to be: we have great difficulty with this with, e.g., huckster politicians, since we might see that what the huckster is saying cannot be true and that the huckster benefits from it and still not be able to show that the huckster knew what they were saying was false when they said it.

Let's make tboyd's scenario rather more concrete: suppose the huckster mind-uploader tilts the marketing at a particular religious group that happens to be wealthy. He uses the group's theological terms in their marketing, which is based on a notion of soul quite similar to mainstream Christianity, although the group is more transhumanism-friendly than, e.g., the Roman Catholics, and advertises the dubious product in their newsletter. You are a former software engineer who is studying to become take orders in this group and are convinced the product cnnot do what it claims. How do you go about showing this?