If you have a flat foot, you would know it by just looking at it. Chances are that if you are hanging out barefoot with your friends who have arched feet, they are going to point it out to you anyway. Nonetheless, people with flat feet can know that they have a flat foot by observing their feet when they are standing. If the entire sole touches the ground, you have flat feet. If there is an arch visible, you have arched feet.