An echocardiogram a noninvasive test for atrial fibrillation, is performed only when the physician needs more data to determine causes of your rapid and irregular heart rate. An echocardiogram can be useful and effective in identifying heart problems that may cause atrial fibrillation. 
Test Description:
An echocardiogram involves placing a probe that gives off sound waves on your chest. This probe also reads the sound waves that bounce from your heart. The sound waves are then converted by a machine into a video. Using this video, your physician will be able to see what causes the rapid and irregular beating of your heart which is a precursor for an atrial condition.
An echocardiographic imaging of your heart can help your physician assess your cardiac chamber function and sizes, the valvular function and pericardium. The information that your physician can get from this procedure may help him in identifying the etiology of atrial fibrillation.
The procedure can also help your physician determine if you are at risk for thromboembolic complications of atrial fibrillation.
Transesophageal Echocardiogram (TEE) for Atrial Fibrillation:
This special type of echocardiogram is done when your physician wants to see the back of your heart. This minimally invasive procedure is done only when your physician wants to have a clearer picture of the heart which is sometimes not provided by a standard echocardiogram. The procedure involves inserting a probe down to your esophagus. The esophagus is located right behind the heart and serves as a passageway that connects the mouth and the stomach.
As this procedure is used to get a clear view of areas of the heart that are not usually seen in a standard echocardiogram, TEE is an effective way to identify an arrhythmia and its underlying causes. 
Studies conducted by researchers at the Stroke Prevention in Atrial Fibrillation (SPAF) showed that TEE is useful in predicting thromboembolism. It is advisable then that if you are showing the first episode of atrial fibrillation, you undergo a TEE evaluation of your left ventricular systolic function, left atrial size and mitral valve function and morphology.
Because of its recognized clinical value, the use of TEE has increased, particularly in detecting sources of embolism and guiding direct current conversion.
In summary, echocardiography is one of the effective tests to identify atrial fibrillation, the most common arrhythmia encountered in clinical practice. Not only echocardiography plays an important role in assessing the fibrillation problems, it is also useful in guiding the proper course of management of atrial fibrillation.