Do not be deceived: God cannot be mocked. A man reaps what he sows. The one who sows to please his sinful nature, from that nature will reap destruction; the one who sows to please the Spirit, from the Spirit will reap eternal life. (Galatians 6:7-8)
How can we be sure that the one we have led to the Lord is truly saved? The truth is, we can't be absolutely sure. Good works don't always indicate a changed heart.
Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. (Matthew 7:21)
We can look for outward signs, evidence of fruit, and the like, but it is not our responsibility to judge another's heart. Just as we must not be quick to condemn, we must not take it upon ourselves to grant assurance of salvation.
We can point them to scripture, speak the Truth in love, and pray that the Holy Spirit will bring conviction or assurance. Beyond that, it's out of our hands.
Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling. (Philippians 2:12)