You see the absolute loss of interest in the face of the person to whom you are speaking. I know how I look when I'm on the other side of this worn-out coin. I always look down at my shoes. Others have different methods: Some look to the right. Some to the left. But the message is as clear as if a light bulb just went off over both of your heads. Interest has left the building.
The key to this whole situation is to realize when it's happened, if you are the one speaking. Don't try to regain the momentum. It's so dead and gone now that it won't be seen in a remake of "Zombies From the Lost Conversations." Just take your medicine and say, "So, what have you been up to?"
Or something to that effect.