Weber's test uses a vibrating tuning fork to determine hearing loss. The fork is struck and held against either the forehead or teeth (in my experience, the latter is much more audible, but also gives you a headache). The patient is asked where the sound is loudest. A normal result is where the sound is heard in the middle of the head. If the sound is louder in one ear than the other, it shows unilateral conductive loss of hearing. A sound in only one ear shows sensorineural hearing loss.
Information gathered from a childhood scattered with hearing tests and long hours reading leaflets in the waiting room.