Christians do have knowledge about the conditions which determine why a person goes to heaven or hell. Namely, a person who sins against God is condemned; if that person dies in a state of condemnation, they will go to hell. A person who genuinely repents and believes the gospel is no longer condemned and will go to heaven.
Christians do not have knowledge of a particular person's beliefs at the time of their death. This means that a Christian would have a hard time claiming that a particular person will go to hell, especially if that person is still alive with time to potentially be saved. It would be illogical to say that a person who is alive has the potential to be saved, but they are definitely going to hell.
What a Christian can say is that if a person dies under condemnation of sin, and they do not repent and believe they gospel, then they will go to hell. This avoids claiming knowledge of the future.
A Christian can and should try to determine if people are currently under condemnation, not who will ultimately end up in hell.