How do you know someone is in heaven? How do you know that a person is in heaven?
I just read the very thoughtful answers/comments to a question about praying to someone outside the trinity and have a follow up question.  Most of the comments assume that you are asking a person who is in heaven to intercede for you. In spite of any evidences we see while people are living, only God knows the heart of man when it comes to someones motivations, beliefs, faith-in-action.
How do you know someone is in heaven?
The short answer is that it is impossible to know!
Generally speaking, we can not tell one way or another, unless God reveals this information to us in some form or another as He deems fit.
As a Catholic, we have a process of canonization to help determine is someone is in Heaven. The ultimate proof is that the Church requires a miracle, to be done through the intercession of one believed to be in Heaven.
St. Thomas Aquinas has a few words on true and false miracles.

Whether the wicked can work miracles?
Some miracles are not true but imaginary deeds, because they delude man by the appearance of that which is not; while others are true deeds, yet they have not the character of a true miracle, because they are done by the power of some natural cause. Both of these can be done by the demons, as stated above (Article 1, Reply to Objection 2).
True miracles cannot be wrought save by the power of God, because God works them for man's benefit, and this in two ways: in one way for the confirmation of truth declared, in another way in proof of a person's holiness, which God desires to propose as an example of virtue. On the first way miracles can be wrought by any one who preaches the true faith and calls upon Christ's name, as even the wicked do sometimes. On this way even the wicked can work miracles. Hence Jerome commenting on Matthew 7:22, &quot;Have not we prophesied in Thy name?&quot; says: &quot;Sometimes prophesying, the working of miracles, and the casting out of demons are accorded not to the merit of those who do these things, but to the invoking of Christ's name, that men may honor God, by invoking Whom such great miracles are wrought.&quot;
In the second way miracles are not wrought except by the saints, since it is in proof of their holiness that miracles are wrought during their lifetime or after death, either by themselves or by others. For we read (Acts 19:11-12) that &quot;God wrought by the hand of Paul . . . miracles&quot; and &quot;even there were brought from his body to the sick, handkerchiefs . . . and the diseases departed from them.&quot; On this way indeed there is nothing to prevent a sinner from working miracles by invoking a saint; but the miracle is ascribed not to him, but to the one in proof of whose holiness such things are done.

Some go out of their way to ask such people they believe to live very holy lives these types of questions. See: here.
The Curé d'Ars (Jean-Baptiste-Marie Vianney) had the following written down in his biography about a man who had committed suicide:

&quot;...a woman...told....Vianney that she was devastated because her husband had committed suicide. She wanted to approach the great priest but his line often lasted for hours and she could not reach him. She was ready to give up and in a moment of mystical insight that only a great saint can receive,...Vianney exclaimed through the crowd, “He is saved!” The woman was incredulous so the saint repeated, stressing each word, “I tell you he is saved. He is in Purgatory, and you must pray for him. Between the parapet of the bridge and the water he had time to make an act of contrition.”

As for the opposite, the Catholic Church has not named a single person as having been known to be in hell. Guess it is best not to know.
Here is a little lighthearted story on this.

Biagio Martinelli(Cesena 1463 – Rome 1544), better known as Biagio da Cesena (meaning &quot;from Cesena&quot;, his native city), was a 16th-century Italian official who served as Papal Master of Ceremonies. He is widely known for his negative reaction to the nude figures presented in Michelangelo's painting of The Last Judgment.
In 1518 da Cesena became Papal Master of Ceremonies to Pope Leo X. He would also act in this role to Popes Adrian VI, Clement VII, and Paul III.
After the completion of The Last Judgment da Cesena said of the fresco, &quot;it was mostly disgraceful that in so sacred a place there should have been depicted all those nude figures, exposing themselves so shamefully&quot;. Da Cesena went on to say the painting was more suitable &quot;for the public baths and taverns&quot; than a Papal chapel. In response, Michelangelo worked Cesena's face into the scene as Minos, judge of the underworld with donkey ears, while his nudity is covered by a coiled snake. It was widely said that when Cesena complained to Pope Paul III, the pontiff joked that his jurisdiction did not extend to hell and the portrait would have to remain.