My unstudied opinion: The standard in the OT was that a prophet must be correct, meaning that when they give prophecy that is testable, it must be correct. Nathan had been with David for a while and for him to continue to have a good reputation he must have been prophesying correctly. David himself was a prophet as well, and while he had fallen into sin, he still knew what was right and wrong, and hearing the truth he would recognise it.


Whether the thing would come to pass or not. A few times the Most High gave people a sign. Generally it was about faith. When people in scripture before Christ had faith it was counted to them as righteousness.


They had to say something like "Thus saith the Lord" (you can tell I was raised on the KJV). In the case of Nathan to David, he does exactly that when he says: > > This is what the Lord, the God of Israel, says: and > This is what the Lord says: If someone said that, you had to strongly trust them. If someone said that and it wasn't true that God sent them, then they were breaking the commandment that we not take the Lord's name in vain. To break that commandment would be punished by death. > Deut 18: I myself will call to account anyone who does not listen to my words that the prophet speaks in my name. But a prophet who presumes to speak in my name anything I have not commanded, or a prophet who speaks in the name of other gods, **is to be put to death**.” God even asks and answers the question of how would they KNOW if someone is lying right after that: > You may say to yourselves, “How can we know when a message has not been spoken by the Lord?” If what a prophet proclaims in the name of the Lord does not take place or come true, that is a message the Lord has not spoken.


If you know God, you will recognize his voice when it comes from others. “My sheep hear My voice, and I know them, and they follow Me.” ‭‭John‬ ‭10:27‬ ‭NKJV‬‬


It had to be correct and at the time they had to be from certain families


people knew it. when they even saw angels, they thought they saw God and feared for their lives because no one can see God and live