Whats the line that separates mental illness...

..., chemistry and environmental issues from actual demonic influence?

All I can say about that is discernment critical to have in moments determining if it's one or the other. For the sake of individual and those close to them it may be best to first assume that the affliction is due to mental illness or stress.

True

I think today it’s hard to even see enough mentally ill people to be able to make the comparison. Most of the ones you would wonder about are institutionalized.

In terms of diagnosis, or symptoms, or treatment?

I guess if you pray for the person, and they get increasingly agitated, you could conclude demonic influence to some degree. There is likely some overlap though, and I suspect that demons prefer to be diagnosed as "illnesses" and will deceive others to disguise their presence.

Your interpretation of the words 'mental illness' must be quite skewed to think this. A large portion of the homeless population in the US, for example, is suspected to be mentally ill. What about the prevalence of PTSD among soldiers? Even those with a mental illness that has a potential to lead to violent behavior are not inherently violent, rather they have stress triggers, and if not triggered will never become violent.
But, when i read the OP, i didn't insert an assumption of what kind of people were referred to, but rather took it as a broad question, the way it was posed. Some see depression, for example, as demonic influence, while others recognize the chemical or environmental influences. This would not require institutionalization, yet it's not uncommon at all.

An ill mind would be very a fertile opportunity for demonic activity and I'm sure many psychotics and people with Dissociative personality disorder may be demon possessed. The problem may have existed before the demon moved in. After the demon is gone they still need love and attention. Also medication may be useful and a wholesome environment or else like the Lord said, the demon will return and after finding the place clean he will invite 7 more demons and leave the person worse than before. A fresh approach must be taken not to skip over pre-existing problems that lead the mentally back like dogs to vomit.

Only someone who has the indwelling Holy Spirit, the gift of discernment, and a close walk with Christ would be able to distinguish one from the other. We have an incident in Scripture where unsaved Jews were attacked by an evil spirit presumably because the Holy Spirit was absent from their spirits.

ACTS 19
13 Then certain of the vagabond Jews, exorcists, took upon them to call over them which had evil spirits the name of the Lord Jesus, saying, We adjure you by Jesus whom Paul preacheth.
14 And there were seven sons of one Sceva, a Jew, and chief of the priests, which did so.
15 And the evil spirit answered and said, Jesus I know, and Paul I know; but who are ye?
16 And the man in whom the evil spirit was leaped on them, and overcame them, and prevailed against them, so that they fled out of that house naked and wounded.

Even though they pronounced the name of the Lord Jesus Christ, they were unsuccessful. That should be a lesson.

At the same time, mental health specialists today are totally secular and dismiss the spirit world altogether as a serious influence. Therefore they talk about chemistry and use drugs on their patients. So it would really take a mature Spirit-filled Christian to properly address these issues (which may be almost impossible). Mental health is ultimately spiritual health, and the first thing that should be addressed is whether on not that person has been regenerated.

This is going back some years, but I read an article once about a psychiatrist who said mental illness isnt caused by chemical imballance in the body.
I wish I could remember this guys name. He said he used to prescribe medications for his patients, but then was diagnosed with a depression disorder himself. He was then treated with medications himself.
He discovered there is no proof any mental disorder has a medical reason. He said meds only treat symptoms, not the cause.
He was healed by the love and understanding of his family. I wish I had saved his story, but there is a lot of information on line to support his claims.

Ask El-Shaddai! He knows, concerning "chemistry" issues.

Don't forget drugs, lead, mercury, HC and other mind altering toxins that leach into our food and air. Airborne lead has decreased 98% since 1980 due to banning leaded fuel and enforcing airborne emissions control standards in the US. Mercury both airborne and in our sea is on the rise... And on our dinner tables.

Is there a difference? I think demons are mostly our fallen flesh

Demons are unclean evil spirits, NOT our human fallen flesh. They are actual spirit beings.

Ok wow... Let me be more clear. I think that what people (church types) call demons, are mostly their own flesh. In other words they are not willing to accept responsibility for their actions not confess their desires, so they call it demon oppression. I forgot I was talking to churchy types and that means it has to be spelled all the way out, because speaking regular language is not acceptable.

Come on bro, that's not a cool or appropriate response. You're the one who misspoke in your first response, own it.

Is he a physician?

Utter nonsense and is in no way Biblical or Christian.

I think there are two words that must be remembered, oppression/possession. There is a difference.