One of the criteria that the prudence of the exorcist will take into account when discerning whether or not someone is being harassed by Satan is the ability of the candidate for exorcism to discover the reality behind known unknowns or even of unknown unknowns.
You just can’t beat the absolute logic of Donny Rumsfeld as United States Secretary of Defense during a press conference at NATO Headquarters in Brussels back on June 6, 2002 (bracketed numbers added):
Now what is the message there? The message is that there are no “knowns.”  There are things we know that we know.  There are known unknowns. That is to say there are things that we now know we don’t know.  But there are also unknown unknowns. There are things we do not know we don’t know. So when we do the best we can and we pull all this information together, and we then say well that’s basically what we see as the situation, that is really only  the known knowns and  the known unknowns.  And each year, we discover a few more of those unknown unknowns.
Some points to take into consideration:
(A) The human spirit is amazing for its sensitivity to others. It is not terribly uncommon that a mother of a son just killed in battle on the other side of the world will immediately know this has happened, even if it takes weeks for the visit to the front door. She is not surprised at the visit, and can speak of the day and time before she is told anything. Similarly, twins are amazing, so tied are they into each other. And when I was a little kid, I could get pretty much anyone in the school library, however far away, to turn around to see who was looking at the back of his or her head. It would only take about ten seconds. What a distraction for those around me, laughing at my shenanigans. I could go on, but you get the idea. This kind of thing is not the interest of the exorcist. These are the kinds of things that we know that we know about the human spirit. Nothing spooky here.
(B) The human spirit can sometimes be despicable for its insensitivity to others. I once lived in a town with a multitude of “charismatic” prayer groups which would pray for others in town. I was told by I don’t know how many people on how many different occasions that, during such “prayer” sessions, one after the other, this or that individual would have a “word of knowledge”, a “prophesy”, which would then be voiced in the form of a petition for which others were to pray. For instance, a word of knowledge might be that so-and-so was right now in the act of adultery. Sigh. So-and-so was not in the act of adultery, but with this voiced (and many other dozens like this about people all over town), people’s good names were continuously being destroyed, even while the “prophets” congratulated themselves for being such great “prayer warriors”. Idiots. This kind of thing is not the interest of the exorcist, except that he might want to slap such people for being such idiots. Yeah, maybe Satan manipulates some of them into such idiocy. More on that a bit further below…
(C) Satan can’t know the future, but can guess at it, just like we can. Surprisingly accurate predictions are not the interest of exorcist, unless there are extenuating circumstances. For instance, Satan’s been around a long time, and knows about the course, say, of cancer in a person of any given stamina, and could easily predict the day on which that person will die. Many people also have a sense of when they are going to die, and can even see the signs in others. So, no big deal. It might be a bit odd, however, if someone were to predict the precise number of deaths – to a man – in an earthquake of whatever metropolitan area around the world five minutes before it happens, again and again and again, with no wrong guesses being made. Satan could make an excellent guess. A human being couldn’t begin to do such a thing. This kind of discovery of unknown unknowns might put an exorcist on guard. But this would only possibly be one item to consider.
(D) Just because we can discover the reality of otherwise known unknowns by way of the human spirit (see “A” above), this does not mean that particularly sensitive people are not being harassed by Satan. Here’s an anedote to illustrate this, with details changed: A young man, an orphan, institutionalized his whole life, now in his early twenties, rather retarded, was recently used a number of times by a certain police unit of a certain country to aid in their investigations as to where the bodies of murder victims might be found. It must have been the volunteers at this institution who brought the news of his “talents” to the police. The head physician responsible for the young man was happy enough to let him be used by the police, thinking that it might do him some good to be involved in anything whatsoever. The young man successfully helped them with their more recent cases, so they gave him their most difficult file, one which was many years old, but still open for political reasons. They only had a picture of the murder victim taken by the murderer, who left the picture in the place of the murder, but then removed the body and went into hiding himself. They thought that if they could find the body, they might also find the murderer. Possible life insurance payouts were at stake. A couple of detectives got in a police cruiser with the young man and brought him out on the highways. This was his first time out of the institution. He told them to go here, then there, it all seeming to be a wild goose chase. But then he cried out for them to stop. They had been driving by untold miles of marshland, every section of which looked like every other section, with no landmarks. They followed him on foot into the marsh, thigh deep in brackish water, wending their way though reeds that towered over their heads. They got out about half a mile into such territory and truly began to fear that they themselves would have a hard time finding their way out. They insisted they turn around, but the young man forged ahead, saying that there was more than one body. They gave him another couple of minutes, at which point the young man reached below the water and pulled up a body part. Then another. They left the marsh by way of a pontooned helicopter, which had carried forensic specialists and equipment to the scene.
Now, such things might seem nice, helpful even. Why would Satan bother. Surely this is all just another bit of the human spirit. Maybe. However, in this case, the young man began to feel oppressed by all this, terribly oppressed, maddingly oppressed. When he was told that it was a “gift”, that he was “gifted”, that he was doing so much good for people with his gift, he cried out that it wasn’t a gift. He hated it. He wanted them away from him. After many other criteria of discernment were considered, an exorcism was done, and he felt absolutely at peace, liberated, and looked it. I’m free, I’m free! he kept repeating. I feel so light! Thank you! It seems some of his clouded mind was cleared up as well. He was just so happy not be have such a “gift”.
Remember the account of the Python Lady? Acts 16,14-40 (NAB)
One of them, a woman named Lydia, a dealer in purple cloth, from the city of Thyatira, a worshiper of God, listened, and the Lord opened her heart to pay attention to what Paul was saying. After she and her household had been baptized, she offered us an invitation, “If you consider me a believer in the Lord, come and stay at my home,” and she prevailed on us. As we were going to the place of prayer, we met a slave girl with an oracular spirit, who used to bring a large profit to her owners through her fortune-telling. She began to follow Paul and us, shouting, “These people are slaves of the Most High God, who proclaim to you a way of salvation.” She did this for many days. Paul became annoyed, turned, and said to the spirit, “I command you in the name of Jesus Christ to come out of her.” Then it came out at that moment. When her owners saw that their hope of profit was gone, they seized Paul and Silas and dragged them to the public square before the local authorities. They brought them before the magistrates and said, “These people are Jews and are disturbing our city and are advocating customs that are not lawful for us Romans to adopt or practice.” The crowd joined in the attack on them, and the magistrates had them stripped and ordered them to be beaten with rods. After inflicting many blows on them, they threw them into prison and instructed the jailer to guard them securely. When he received these instructions, he put them in the innermost cell and secured their feet to a stake. About midnight, while Paul and Silas were praying and singing hymns to God as the prisoners listened, there was suddenly such a severe earthquake that the foundations of the jail shook; all the doors flew open, and the chains of all were pulled loose. When the jailer woke up and saw the prison doors wide open, he drew (his) sword and was about to kill himself, thinking that the prisoners had escaped. But Paul shouted out in a loud voice, “Do no harm to yourself; we are all here.” He asked for a light and rushed in and, trembling with fear, he fell down before Paul and Silas. Then he brought them out and said, “Sirs, what must I do to be saved?” And they said, “Believe in the Lord Jesus and you and your household will be saved.” So they spoke the word of the Lord to him and to everyone in his house. He took them in at that hour of the night and bathed their wounds; then he and all his family were baptized at once. He brought them up into his house and provided a meal and with his household rejoiced at having come to faith in God. But when it was day, the magistrates sent the lictors with the order, “Release those men.” The jailer reported the (se) words to Paul, “The magistrates have sent orders that you be released. Now, then, come out and go in peace.” But Paul said to them, “They have beaten us publicly, even though we are Roman citizens and have not been tried, and have thrown us into prison. And now, are they going to release us secretly? By no means. Let them come themselves and lead us out.” The lictors reported these words to the magistrates, and they became alarmed when they heard that they were Roman citizens. So they came and placated them, and led them out and asked that they leave the city. When they had come out of the prison, they went to Lydia’s house where they saw and encouraged the brothers, and then they left.
The Lord always wins these sort of things. We only have to remain faithful. Winning might mean being a martyr, but that is really winning, isn’t it? Yep. That’s when we discover the greatest unknown unknown, that is, that which eye has not seen, ear has not heard, nor has it entered the heart of man the great joys that await us in heaven.