Use masking tape to tape one end of each of three long strings to the bottom of the mirror.

Back up from the mirror and hold one string up to your eye. Have your friend hold a second string horizontal to the floor and tape it to your body where it hits. Standing straight up, look in the bottom of the mirror and have your friend tape the third string to the lowest point on yourself that you can see. (Leaning forward can affect the outcome so try to stand straight up.)

Have your friend carefully measure the distance from your eye to the second string and from the second string to the third string. Are the measurements close to being the same? (They should be!)

Back up and try it again.
Did these measurements change?