You could do a raycast when falling through the air and get the height from which you are falling, if its greater than a certain amount, then you can call the KO of the character.
The raycast would not need to be polled every frame as it would since your falling, the height will never go higher. But if there was a cliff or overhang that sloped, then you would need to know the distance.
A poll every 10ms -100ms or more would probably be fine.
Hope this helps
