Thank you for a wonderful post, simply.
BDSM stems from love and care, abuse comes from hate and rage. BDSM is about the person you are with, abuse is about just having a human punching bag.
How do you spot the difference? The look in his eyes. A Dom never gets that glazed raged look, his face is never contorted in fury, contempt, disgust. A Dom never "handles" his sub when drunk or angry. Doms movements are controlled and purposeful, he doesn't simply pummel you to death. And he certainly doesn't make a daily habit of it.