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.