You have a glass of water that looks about half full. How can you tell, only using the glass of water itself, if the glass is half full or not? The glass is a right cylinder.
Tip the glass of water until the water reaches the rim of the glass and if the water lines up perfectly with the bottom rim of the glass, it is half full.
If the water lines below the bottom rim of the glass, then it is less than half full. Likewise, if the water lines above the bottom rim of the glass, it is more than half full.