You mention the presence of enlargement only on the left side. It is not at all unusual to have enlargement on one side only or one side considerably larger than the other.
The problem is your wanting a test to find out why it is there. It is possible that you may never know and it is highly possible that there is nothing at all wrong with you at this time.
One of the most common explanations used for Gynecomastia is the word "Idiopathic." I've heard the word defined as "A medical word that means your guess is as good as mine".
There may have been an event in your past that caused your hormones to be out of balance for a while. Even though the event is no longer there, the effect remains.
All this presumes that your condition has not changed in some time. If your breast enlargement is very recent or even ongoing, you may need to see an Endocrinologist to find out what is going on.