Right there on the curve

While driving back from work, right there on the curve of the road, I saw a guy very jumpy and moving rapidly in a car driving towards me, I thought they must be guys probably enthusiastic about something. And suddenly I made out what his movements were, I looked at the person sitting on his side, and found a woman trying to be as far away as she can from him, clinging to the door as much as she can, and her arm placed in front of her face... defensive position.

And then as my car got closer to theirs, there he was at it again, he held his arm up high too fast and hit the air beside him, because she was as far as she can be, and he wasn't able to focus on hitting her and driving on a hard left. I know he saw me looking, the dude with the glasses, ugly as a monkey butt.

I slowed down, I was shocked, I saw from the bits of her face that was hidden by her arm how scared she was, how much of an animal he is. I didn't know if there was anything I can do, but seconds later, they were gone. Out of my life. I was going back home to get ready for going out, and she... she was living one of her worst moments in life.

I always wonder, will I be able to make out if a man is abusive? How can you know if your friend, boyfriend, colleague.. is abusive?