If you take people and their deceptively simplistic opening words at face value I suggest you are missing out on perhaps 70% of what is really going on in their hearts and minds.
The poster wants to understand how we know with reasonable certainty that God backs what Man says:
It sounds like they are ASSUMING this is true rather than actually having this CONFIRMED by God.
One must use the words presented, and if there is an ambiguity, inquire.
We know because of the promised guidance of the Holy Spirit that the Church has received.