"How can you tell if someone has a dark spirit, demonic or otherwise?"
I use the term ‘demon’ to categorize most dark entities. While I don’t believe in Satan/Lucifer/Hell I do know that dark energies exist and that their manifestation in and around a person, place, or item can be devastating.
Demonization (or the possession by a dark spirit) can be broken down into several stages (originally categorized by Edward and Lorraine Warren) are:
Infestation- The presence of a dark energy on someone/thing is considered an infestation. This can happen because of someones lack of care in using a divinatory tool or tapping into otherworldly energy without proper safety, and in the acts of the naive it can be done on purpose.This is first noted by external factors, movement of inanimate objects without an apparent source, phantom sounds or voices, changes in temperature or the changes in animals. To counter Infestation I suggest cleansing with White candles, Salt, Dragon’s Blood, Olive Oil, and Sage to banish the spirit. This is the demon at its weakest.
Oppression: Often noted by their paranoia and stress levels in a person. Are they eating less? Sleeping less? Are they angry/sad/scared without a cause? Oppression is the feeling of dark energy as it drains you and others. It is a weakening tactic. This is done by the dark energy to break them which will make possession easier. In this you must call upon a group to cleans the home and the person. This is not an exorcism, rather a group cleansing. Use all four elements, call on the God/Goddess and do a full cleansing of the house.
Possession: Once the mind of the target it broken (or the demon has utilized itself in such a way that it can taint an item or an area) the entity has achieved possession. This can be noted by a sure and sudden change in the target. Where once they were paranoid and frightened they now exhibit little or no emotion at all. Sometimes they lash out violently and may speak gibberish. In a possession scenario an exorcism is just about the only cure.
Please keep in mind that while the Catholic Church is the only religion to have a school of exorcism they are certainly not the only faith to practice the act. Nearly every faith out there has a ritual for the banishment of energy.
Now, keep in mind that in many cases there is no dark energy. In many it is mental illness. Do not make the mistake of mistaking one for the other.