Hi! 'DEADn'
Your question is an intriguing one.
Well, how 'bout yuh play a self-answering quiz?
If we cut someone into microscopic morsels, would we find a thought? Would we find the prompting behind their negative behavior? Would we find the origin (not the symptoms) of their sickness? Would we find residues of guilt? Some tell-tale signature of love or hatred?
If you answered, "No", you would be correct.
That's because only the visible can be detected in this earthly dimension.
The unseen human spirit of a man ("hidden man of the heart") is invisible and belongs to the spiritual dimension.
Everything that becomes visible on the earth has its origins from the invisible realm.
For instance, a thought.
A thought originates in the invisible world. It arrives as an inspiration to the human spirit; at which point, the person has the choice to take ownership of the thought and acting on it or not.
The Garden of Eden is an illustration of this activity. There were two trees in the Garden. In the same way, there are two centers in the invisible world; the kingdom of light and kingdom of darkness. There is a constant flow of inspirations from these two 'trees' which every man is having to choose between. Inspirations of peace, joy and righteous (etc) or hate, covetousness, anger (etc).
Personally, I believe that the absolute term 'demonic' or 'satanic' has been much abused and misunderstood by a lot of people. There are different layers or hierarchy in the kingdom of darkness.
For instance, there is a difference between "the prince of the power of the air, the spirit that now worketh in the children of disobedience" and those described in Ephesians as "principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places".
Regardless, what we perceive in this visible world, (eg: everything we see manifested in the physical body of a person), originated first from the invisible world. In that regard, our physical bodies mirror realities from out of the unseen world.
We could perhaps say that the human being behaves similar to a television set. If we ripped the TV set apart into tiny pieces, we would never find the news anchor which was displayed on the screen. Nonetheless, the person of the news anchor exists; albeit hundreds of miles distant in a broadcast studio.
The reason we see the image of the distant news anchor on the television screen is because the antenna of the TV set received the transmitted signal which was then subsequently decoded and displayed as an image.
In a similar manner, the human spirit of a person behaves like an antenna and, depending on which spiritual kingdom it is connected / tuned into, the inspirations from that realm will be displayed within the physical presence of the person.
The reason we might see a person visibly bent over with unseen psychological burdens is because he is connected to invisible powers of darkness ... unseen inspirations of depression.
So, the answer to your question: "Where do we draw that line between the working of satan vs our own decisions?" can only be answered by discerning the visible fruits which indicate the kingdom their human spirit is receiving inspirations.