reality

You can tell what reality is because you can feel it.  Yes, you can feel the terror of a nightmare but in most cases you can't even remember what it was that scared.  You can feel reality physically and emotionally.  You feel the fur on a puppy and you also feel the companionship between the two of you.  When reading a book you can only feel the emotions portrayed and visualize what it is like but in reality when there is a scene filled with drama you feel the emotional tension and see exactly who the problem is with because you are a third person just watching/visualizing.  You can know what is real by how you can feel it.