The bible describes demons as spirits. One cannot see spirits. But one can SOMETIMES identify their presence "by their fruits." That's a bible quote.
 What are "fruits" as used in the bible quote ? It's a symbolic (metaphoric, parabolic) use of the word. "Fruits" in this context are RESULTS or EFFECTS generally speaking.
 Want a specific application for that general knowledge ? OK look at the world we live in. See wars, conflicts, crime, hate, poverty, ignorance, lack of love ??? Bible says there's a spirit for that. It's called Satan and his helpers or "demons."