The next time you're alone in your room, turn down the lights.
Think of something on your body that varies in length, such as hair. It must be clearly viewable from your perspective.
Grab a ruler and, looking in the mirror, quickly grab a hair at random; you must not confuse it. Hold it in position as best you can and note the length. Look down. Yours will be different.
Don't look back up.
Don't turn your back to that mirror ever again.