How do flight attendants know seat belts are fastened?
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.