How do you know when one has a demon?

Answer 1

The easiest way is to realise that possession by demons is not a scientific fact. It does not happen.

The more dramatic way is that employed in modern times by the Catholic Church before engaging in a rite known as exorcism. First, the supposed victim is put through a series of tests to establish whether he or she is suffering from any form of illness, including mental illness. Only then will a priest agree to perform an exorcism. Of course the preliminary tests must not be too rigorous, otherwise the Church would never get to perform exorcisms at all.

Answer 2

It depends on which world lore this demon arises from.