Should such a leak occur, I would just expect China to adjust their numbers and say, “Woops, we missed that one!” It’s not really going to prove anything, if you’re willing to buy into what China says post-hoc.
Realistically, to absolutely clinch things, you would need a leak (or recording) from someone in the leadership revealing an active intent to cover up and lie. Minus that and regardless of what information comes out - from the government or individuals - it could always be explained away through incompetence, data loss, confusion, or other things.
But we can catch them in lies through other means.
For example, according to China, Wuhan City had 6.51 hospital beds per 1,000 at the start of things:
That would be about 72,131 beds total.
They then, purportedly, added 11,537 beds via a set of temporary hospitals:
They, supposedly, built one of their temporary hospitals finishing around February 1st:
They also, supposedly, built and closed 16 separate temporary hospitals with the last closing by March 11th, just a little over a month later:
Based on American statistics, your median patient stays in hospital for about 11 days:
https://www.cdc.gov/coronavirus/2019-ncov/hcp/clinical-guidance-management-patients.html
That gives us enough room for about 3 waves of patients to move through Wuhan’s 83.6k beds. Though, let’s say that 30% of them were used for normal, non-Covid purposes. That would mean that we should expect about 175.6k patients.
In the US, if you’re hospitalized, there’s about a 30% chance that you will end up in ICU. From there, there’s about a 50% chance that you die.
Given that China was working, in at least part, with makeshift hospitals, we should probably assume that the fatality rate was higher than 50% among those who went critical. Let’s say 65%.
So 175.6k patients go to hospital, 52.7k go critical, and 34.2k die.
Actual count, according to China: 4,512 for the entirety of Hubei province.
Alright…
Now we’re assuming that there are three waves of ill. What if they were extra nice and let everyone chill in bed for a few weeks after getting better, and only had one wave. 34.2k / 3 = 11.4k. We’re still over double the number that China reported and it’s silly to think that they let people hang out in the hospital during a crisis. At most, we might say that they only had two waves pass through - hence closing the hospitals. So a real minimum from that vantage is 17.1k for Wuhan city.
So let’s say that they didn’t use 30% of their hospital capacity for non-Covid cases. Let’s say that it was 80% of the normal hospitals. And let’s assume two waves. Now we’re looking at 51.9k who get sent to a hospital, 15.6 who go critical, and 10.1k deaths. That’s still over double what we are told.
And, China is currently telling us that many people died at home, because they couldn’t get access to a hospital.
I don’t know if they’re lying about how many beds they had or if they’re lying about the fatality count, but it would make no sense that they somehow were able to get < 4,512 deaths out of 83,600 hospital beds, given what we know about how the disease works.