Yes you CAN know if it's written in the scripture.
I've also seen Christians proclaim things that are not expressed in scripture. But it's rather easy to determine if its them or actual scripture.
But to say we don't know, is not correct. Of course we can know. They have been revealed to us.
But unto us God revealed them through the Spirit: for the Spirit searcheth all things, yea, the deep things of God. For who among men knoweth the things of a man, save the spirit of the man, which is in him? even so the things of God none knoweth, save the Spirit of God. But we received, not the spirit of the world, but the spirit which is from God; that we might know the things that were freely given to us of God. Which things also we speak, not in words which man’s wisdom teacheth, but which the Spirit teacheth; combining spiritual things with spiritual words. Now the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him; and he cannot know them, because they are spiritually judged. But he that is spiritual judgeth all things, and he himself is judged of no man. For who hath known the mind of the Lord, that he should instruct him? But we have the mind of Christ.
Bible Gateway passage: 1 Corinthians 2:10, 1 Corinthians 2:11, 1 Corinthians 2:12, 1 Corinthians 2:13, 1 Corinthians 2:14, 1 Corinthians 2:15, 1 Corinthians 2:16 - American Standard Version
One thing that can be done is to ask the person where thatbis found in scripture. Then you can see for yourself if it's really there or not.
But it's obvious that we can know what God thinks. I think it's clear about many things. For example,
even the righteousness of God through faith in Jesus Christ unto all them that believe; for there is no distinction; for all have sinned, and fall short of the glory of God;
even the righteousness of God through faith in Jesus Christ unto all them that believe; for there is no distinction;
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