How do flight attendants know seat belts are fastened? 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?
In addition to Jamiec's correct answer, I would add that you can tell when a seat belt is not fastened even if the buckle is covered by someone's shirt (or belly!).  You can normally see the segment of belt alongside the person's hip and it's easy to tell if that piece of strap is under tension or not.  Some people actually try to disguise the fact that their seat belt isn't fastened by covering up the buckle, hoping to avoid detection.  By looking for the strap alongside their hip, I can avoid asking to see the buckle itself.  

Of course, if I can't really tell by a quick glance, I ask them to show it to me.  It's very rare that anyone actually takes offense to this.