I'm a simple curious traveller. I always see flight attendants passing by the seats checking whether belts are fastened. They pass quickly and without looking carefully at each single person, to see if the seat belt is fastened.
I stared at them and found them looking at some specific point on the ground that could possibly tell them with a glance, but I couldn't figure out what was that point.
Can somebody explain how do they check the seat belts status?