A physician may become suspicious about the presence of congenital heart disease if the child has one or more symptoms, is not growing normally, or has a heart murmur. Physicians will order tests in order to determine the nature of the problem. The most common tests include:

• Electrocardiogram (EKG). An electrocardiogram (EKG) is a recording of the heart’s electrical activity as a graph on a moving strip of paper or video monitor. The highly sensitive electrocardiograph machine helps detect heart irregularities, disease and damage by measuring the heart's rhythms and electrical impulses.
  
  
• Echocardiogram. This test uses sound waves to visualize the structures and functions of the heart, including the septum (the wall between the left and right sides of the heart). A moving image of the patient’s beating heart is played on a video screen, where a physician can study the heart’s thickness, size and function. The image also shows the motion and structure of the four heart valves, revealing any potential leakage (regurgitation) or narrowing (stenosis). During this test, a Doppler ultrasound may be done to evaluate blood flow. This is usually considered to be the best diagnostic test for congential heart disease. No sedation is necessary and there is no risk or side effect from this test. 
  
  
• Chest x-ray. This test evaluates the size of the heart and the blood flow to the lungs. It also helps in evaluating whether there are any secondary problems within the lungs because of the heart problem.
  
  
• Pulse oximeter. A sensor placed on the tip of the finger or earlobe that reads the amount of oxygen in the blood.
  
  
• XMR. XMR is a new diagnostic technique that combines magnetic resonance imaging (MRI) and conventional x–rays to create a three–dimensional image of the heart, greatly improving a physician’s ability to measure blood flow and tell how the heart is beating. Imaging tests such as an MRI usually require significant sedation in the case of infants and young child since the patient must stay perfectly still during the imaging.

The physician may also need to do a more invasive test that uses cardiac catheterization. This is a procedure that requires thin tubes (catheters) to be fed from blood vessels in the arm or leg, up into the heart. The catheters measure the pressures in the different heart chambers and can detect mixing of blood between the two sides of the heart. Pictures of the heart (angiograms) can also be taken by injecting a special dye contrast medium through the catheters and into the heart and great arteries. Certain congenital heart lesions can be treated using special catheters (interventional catheterization).

In some cases, CHD can be diagnosed while the infant in still in the mother's womb. Studies have found that early detection of CHD in the womb improves the newborn’s chances during surgery to correct the defects.  Prenatal detection of hypoplastic left heart syndrome (HLHS) and other severe forms of CHD can allow parents to receive counseling, have more time to consider their options, and plan to give birth in a facility best equipped to treat their infants.