For the carnal minded natural man, it is his working assumption that something can only be known via natural means i.e. the sensory perception processes that occur in his brain.
For the spiritual man things can be known in the above manner as well as in an additional manner which takes place in his spirit. The faculty of the regenerate man with which he assimilates spiritual truths enables him to know spiritual truths whereas the carnal or natural man is unable to due to him being spiritually dead.
So sir, when you, a natural carnal man ask how we who are spiritual can know something which takes place in the spiritual realm, you are asking for us to give you an answer that is natural. The answer is not a natural answer at all but a spiritual one. So you being carnal are unable to even understand or discern the concepts about which we speak.
1 Cor 2:9 But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him.
1 Cor 2:10 But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
Cor 2:11 For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God. .
1 Cor 2:12 Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God. .
1 Cor 2:13 Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual. .
1 Cor 2:14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.
1 Cor 2:15 But he that is spiritual judgeth all things, yet he himself is judged of no man.
Pay close attention to the underlined portion of the above text. It explains my point.