But if you are convinced the Chinese government did know, or knew more then they told the world, then that would be easy to prove.
Hire and fire
Have a good look at the most successful Chinese companies active abroad, and examine their hiring and firing over the last twelve months. You can safely assume they will have to deal with some governmental controls, or at least will be provided with some governmental information.
If they suddenly have changed their policies, and have been actively firing their older employees (let's say anyone above fifty, knowing those would be more susceptible to Covid 19)... Well... If so, then there's at least some reason for suspicion.
Me? I wouldn't have a clue. I'm just a stupid dad trying to live with this new reality.
(Regardless, I'm convinced the more totalitarian regimes will have a field day with all of this.)
And, obviously, Trump claims something similar. Unfortunately (fortunately?) when he claims something it probably isn't true.
Remember, we're not there yet... But for now, I'm done with everything Covid-19. Getting tired of it, like most of us. Time to do some fun things, whilst still being careful.