And last of all He was seen of me also, as of one born out of due time. For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God. But by the grace of God I am what I am… – 1 Corinthians 15:8-10

How do we know that Paul was born again, and that he was called to be an apostle of Jesus Christ?

See how he confessed his sin and forsook it. He that covereth his sins shall not prosper; but whoso confesseth and forsaketh them shall have mercy.” Are you, dear friend; willing now to confess your sin? Do you turn from it with loathing. Do you desire, henceforth, to be delivered entirely from it? Well, then, your repentance is another sure evidence that you are born again. If you have seen Jesus taking your sin upon Himself and suffering its dread penalty; if you have confessed your sin, and by faith laid it upon Him as your Sacrifice and Substitute, you are born again, though you may have been, in a certain sense, “born out of due time.”

I am sure he was converted, also, because he praised the grace of God. Read the 10th verse: “By the grace of God I am what I am.” Even when he truthfully says, “I labored more abundantly than they all;” he humbly adds, “yet not I, but the grace of God which was with me.” It is a sure sign of conversion when a man knows that he is saved by grace alone, and does not, attribute it to his own merit, or his own work, but praises and adores the sovereign mercy and grace of God. Have you that evidence, dear friend? Then are you born aright, even though “born out of due time.” ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/2663.cfm