1. The character has an unnatural eye colour. (Their eyes are completely black, or are yellow with slit pupils, or their eyes are glowing, etc.)
2. The characters' word balloon is jagged and the font is irregular. Most commonly used when the character is speaking, unless you want to be subtle. Their voice may become deeper or more savage.
3. A ghostly figure is behind the possessed character.
4. The character moves in an irregular manner. The character moves in an unusual way, like gliding or hovering above the floor.
5. The characters' behavior is irregular. The character might refer to themeselves as "we" or "us", or they may have access to hidden knowlege or languages.