How are some forms of heart disease diagnosed?

As with most medical conditions, your doctor will obtain most of the information required for diagnosis based on the information and description of symptoms he or she gets from you during the interview. Physical examination and lab investigations help to confirm or rule out the different possible diagnoses.

Listening to the heart through the stethoscope can reveal obvious abnormalities such as murmurs and irregular rhythms. There are other physical findings like extremity swelling, abnormal blood pressure, and fluid in the lungs which can help to diagnose heart problems.

Subsequent tests such as an electrocardiogram (sticky electrodes on your chest, wrists and ankles to check the electrical activity of the heart for a few seconds), echocardiogram (ultrasound scan of the heart), and exercise stress testing (the treadmill test) can also be performed to confirm or rule out serious heart disorders.

some by plain xray like cardiomegally, in blood exams like high sgpt-sgot, thru auscultation like premature heart beats, some
by ultrasound, some by magnetic resonance imaging (MRI),
some by getting the patient's health history.
Please see tutorials on Echocardiography, Stress echocardiography, Coronary angiography / angioplasty and Coronary artery bypass graft.
http://www.nlm.nih.gov/medlineplus/tutor...
http://www.nlm.nih.gov/medlineplus/tutor...
http://www.nlm.nih.gov/medlineplus/tutor...
http://www.nlm.nih.gov/medlineplus/tutor...
Most are diagnosed by obtaining a history from a patient, and most of the remainder are diagnosed after performing a physical examination. The history and examination determines whether or not testing (and what type) should be done and testing obviously will diagnsose those in whom heart disease was suspect after history and examination. Unfortunately a fairly large number of people escape detection until an 'event' such as a heart attack.