How do we know that Paul was inspired and had revelation? Well he tells us. God also gave him the same powers that he had given exclusively to the earlier apostles: signs and wonders, the ability to miraculously heal terrible diseases, and to cast out demons which in those days were many. Paul was given the same gifts as Peter and John and all the other eleven apostles. But the eleven had these powers, and only apostles had them, and the apostle Paul was given the same. Possibly they were also given to some of the apostolic helpers, their immediate assistants, but that is controversial. So because of these attestations from God, his signs and wonders and powers, people knew he was an apostle and inspired.
But how do we know, further, that God inspired him with unique words and seemingly new teachings? Because God not only inspired Paul, but he showed him exactly where these things had actually been taught before in the Old Testament. Paul says nothing actually new. He shows, he reveals only things which were already revealed partially in the Old Testament. If you read Paul’s Letter to the Romans and you have a Bible with marginal references and you follow the references, you will see that the people in Old Testament times were told about this but not so fully. Everything the apostle Paul says through these epistles when he is teaching doctrines with such close reasoning, he backs up with Old Testament references. Now that is a wonderful authentication. People throw these insults at the Bible – Paul invented a lot of new doctrine. No, he did not. For one thing he was inspired of God, for another thing, he had the signs and wonders to prove it. But furthermore God enables him to prove it by giving you a reference for all that he teaches back to its root in the Old Testament.