How do I tell the difference between jinn possession and mental illness?

Honestly, from the outside, you often cannot. And that is precisely why the medical assessment has to come first.

Conditions that closely resemble descriptions of possession include:

• Psychosis and schizophrenia, which produce hearing voices, beliefs others do not share, and behaviour that seems to belong to someone else
• Dissociative disorders, which produce memory gaps, altered identity states and changes in voice and manner
• Temporal lobe epilepsy, which produces episodes of altered consciousness, unusual sensations, automatic behaviour and no memory afterwards
• Severe PTSD, which produces flashbacks, dissociation and reactions that appear entirely disconnected from the situation
• Bipolar disorder in manic phases, which produces dramatic personality change, altered speech and behaviour utterly unlike the person

Every one of these is treatable, and every one worsens without treatment.

The correct sequence is always the same: full medical and psychiatric assessment first, continue any treatment prescribed, and perform ruqya alongside it. These are not competing options, and no scholar has ever held that they were.

If someone is at risk of harming themselves or another person, that is an emergency requiring immediate professional help.