It's a great question. For the biblical people, they seem to have been acutely aware that God was speaking to them and through them.

I think the issue, then, comes in how you or I can tell. In the Bible, they seem to have pursued tests of truth. If a prophet prophesied something, did it come true? The thought was that if it was truly from God, the prophet would bat 100%. Tests of truth include corresponding to reality, complementing (and not contradicting) other known revelations from God (weighing it against already established inspired messages), logical consistency, the person consistently lives what they teach with integrity, and things like that.

So the words of Jesus, Paul, Joseph Smith or Mohammad don't get a free pass. No one gets an automatic golden ticket. All must prove by evidence that their words are revelation.

what I’m looking for is a sort of “ground up” way to determine how we can know if something is revelation.

It seems to me, that anyone who claims revelation, whether it be Jesus, Paul, Joseph smith, Muhammad- they are all, at least initially, on equal footing. So, the question is how might we determine if any or none of these are really revelations?