look over one's shoulder
glance over one's shoulder

He hit me while I was looking back over my shoulder.

She'd taken to looking over her shoulder for him, and didn't like the feeling one bit.

We started off, and I looked over my shoulder at her.

He looked back over his shoulder at the world they had left.

He looked over his shoulder to see how much help he'd get.

He looked over his shoulder at her once, and she turned away.

But I can't be looking over my shoulder every second of the day.

He looked over his own shoulder and saw the thing again.

She got up and came to look over my shoulder.

I looked over my shoulder at the rest of the room.