If you diagnose a disease, you recognize it by the signs and symptoms your patient shows. Your high temperature and the green crud you keep coughing up will allow your doctor to diagnose your bronchitis.
The verb diagnose was first used in medicine, meaning "make a diagnosis," or identify a disease by observation of symptoms. Use it to describe the process of figuring it out. If you have bad gut pain, ask your doctor to diagnose your condition. You can also use diagnose for other problems, like if you diagnose poverty and malnutrition as the real causes of a student's terrible behavior, or diagnose the virus that caused your computer to crash.