How can I distinguish 'you are not going to believe us' and 'you are not going to believe this' ?
Only from the context. (Italian speaker)
I think the context in which it is used would indicate the meaning, It accepted both usages from me.
Question for a native speaker: is "non ci crederai" or "non crederai questo" more commonly used for sharing surprising news?
"Non ci crederai" is usually used for sharing surprising news. "Non crederai questo" is literally correct but not fluid in common speech.
or "you are
not going to believe it"?
Non lo crederai
How do we say "You don't believe it."?
Non lo credi
I believe it'd be: Non ci credi?
How do you know that it doesn't mean "You are not going to believe us." ??
This was answered above.
You will not believe this = Non ci crederai = You will not believe us
Is it just me or did the spoken sentences become a whole lot easier to understand today 16 December, 2020?
It seems like "You won't believe this." would be a more direct translation. My hunch seems to be backed up by context.reverso.net and Google translate.
Not quite, non ci crederai means both "you're not going to believe this/us" and "you won't believe this/us". There is only one way to express the future simple in Italian.