Some common signs recognized by various cultures that a person may be demonically possessed are:
1. Speaking in tongues. That is, the afflicted person speaking in languages they should not know. Sometimes they speak their native tongue backwards.
2. The air around them is incredibly cold, but is a normal temperature everywhere else.
3. They show an aversion to holy items. By aversion, I mean that it burns them and leaves a mark.
4. Strange scratches or bruises appear on them without anyone or anything touching them, sometimes appearing even as other people watch.
5. Animals begin to show fear of the person. They refuse to go near them, and act hostile. However, vermin, such as rodents and insects, seem drawn to them.
The most clever of demons, however, will never let you know that they’re there. If you can detect a demon, it’s sloppy work on their part.