70% of people look to the mirror to determine what they look like. it seems logical to determine how you look based on the reflection in the mirror. However, what you see in the mirror is not an exact replica of what stands before it.
15% of people know what they look like based on what others say about their appearance. The image of the self, and the image others have of us, is part of how we develop our overall self-image. However, remember that opinions are subjective.