I am building a Minecraft survival games map, in which the battle area is on a floating island. My friends discovered that if they are falling to the ground, they can quickly logout and log back in to escape fall damage and not die, thus cheating. 

Is there anyway to detect if a player logs out so I can increment their Death score?

I'm building it vanilla, I'm trying to make a whole map automated with just command blocks and redstone, this is the last thing I need to do now.