Say we do have a 1:1 representation of the human brain in software. How could we know if we're talking to a conscious simulation of a human being, versus some kind of philosophical zombie which appears conscious but isn't?

Without a solid way to differentiate 'conscious' from 'not conscious' any discussion of machine sentience is unfalsifiable in my opinion.

How do you tell the difference in other humans? Do you just believe them because they claim to be conscious instead of pointing a calibrated and certified consciousness-meter at them?

I obviously can't prove they're conscious in a rigorous way, but it's a reasonable assumption to make that other humans are conscious. "I think therefore I am" and since there's no reason to believe I'm exceptional among humans, it's more likely than not that other humans think too.

This assumption can't be extended to other physical arrangements though, not unless there's conclusive evidence that consciousness is a purely logical process as opposed to a physical one. If consciousness is a physical process, or at least a process with a physical component, then there's no reason to believe that a simulation of a human brain would be conscious any more than a simulation of biology is alive.

So, what if I told you that some humans have been vat-grown without brains and had a silicon brain emulator inserted into their skulls. Are they p-zombies? Would you demand x-rays before talking to anyone? What would you use then to determine consciousness?

Relying on these status quo proxy-measures (looks human :: 99.9% likely to have a human brain :: has my kind of intelligence) is what gets people fooled even by basic AI (without G) fake scams.