To know when the gun pointed in your face will actually be able to shoot you.
Looking into the cylinders of a revolver will tell you if it's loaded or not.
Noting the position of the slide of a semi-automatic will tell you if it's in battery or not. (an out-of-battery gun is jammed, and will not fire)
There are other things, like being able to note if a safety is on (red dot means safety is OFF), noting a loaded chamber indicator (not all guns have them), or a dropped hammer on a single-action pistol (they must have the hammer cocked to fire). A few trips to a local range to shoot and familiarize yourself with some common pistols would go a long way.
I'm not saying you should take a guess that the gun in your face is unloaded, or incapable of firing in its current state, but knowledge is power.