You, a right cylinder glass of water, 3 buttons, instructions, and a large flat land that seems to go on forever, are the only things that exist in a universe. In this weird universe, the exact same physics apply. The 3 buttons are labeled "Less", "Half", and "More". The instructions are as follows:
Hello fellow human, I have abducted you from your puny planet. I am done testing you, and have wiped your memory. You have just 1 chance of escape. You must tell if the glass is half full, less than half, or more than half with water. If you are wrong I will remove the ground and let you fall into an abyss forever. I labeled 3 buttons accordingly. I have also given you perfect sight and perfect balance/handling. Please note perfect sight only helps you with levelling; it does not help in originally finding half. Also you do not have any measuring instrument and you cannot spill out or add anything. Good luck... Not.
The glass appears to be half full, but how can you tell for sure?