How do we know if a person is truly repentant? 

Kevin Swanson explains in this video that when someone has made a profession of faith for the first time, time will tell whether a person is truly regenerate and repentant. Sometimes, those who initially appear to be repentant are like the seeds that fell on the stony or thorny ground as mentioned in the parable in Matthew 13. 

It’s good to assume and hope for the best but repentance will manifest itself with the fruits of repentance. 

Matthew 13:7 (NKJV) – “And some fell among thorns, and the thorns sprang up and choked them. 8 But others fell on good ground and yielded a crop: some a hundredfold, some sixty, some thirty. 9 He who has ears to hear, let him hear!”

It's one reason why when we find that somebody has made a profession of faith for the first time, We don't assume that they are regenerated and repentant and they will be glorified for certain. That is not a certainty. Obviously the Spirit of God does work in individuals assuring them of their own salvation, But we usually say time will tell. Now again, we can assume to the point that we bring them into the membership of the church, but oftentimes these people prove themselves to be Stony ground hearers or thorny ground hearers as Jesus presented the parable in Matthew chapter 13. So we have to say time will tell.

We want to be patient. We want to watch for fruit. I think it's good to assume the best and to hope the best, but at the same time we should not be surprised if one person or another person proves to set the wrong trajectory over a period of time. Repentance will manifest itself with the fruits of repentance, And that may not be immediately obvious, so it's good to just sort of hang out and wait and watch, and you'll see that some do exhibit the fruits of repentance and others do not. You