Simple test

Whisper a number, or rub fingers together in one ear while blocking the other ear with a finger.

Rinne's test

Place a vibrating tuning fork (256 or 512 hz) on the mastoid process (to assess bone conduction), then hold just lateral to the external ear. Which is the louder sound? Air conduction should be better than bone. If bone conduction is louder, there is conductive deafness in the ear.

Weber's test

Place a strongly vibrating tuning fork on the middle of the forehead. Is the sound louder on one side, or equal? 

Causes of conductive deafness 

Causes of sensorineural deafness