How do we know if someone has committed the sin unto death?

According to 1 John 5:16, John notes that if a believer sees a brother sinning a sin that is not unto death, he should pray for them. However, for the sin unto death, John advises not to pray for them. The challenge lies in knowing when someone has committed this sin. Unless they die openly rejecting Christ, we generally cannot discern their eternal state. The warnings given in scripture serve to remind us of the seriousness of forsaking Christ and His assembly, reinforcing the necessity of continual faith and obedience to the truth. This fact leads us to approach such matters with prayer and caution.