Ruling Out Light Leakage
This can be done by placing your ring over a collared object. If you see the color through the diamond, and there are dark spots, then you know the stone is leaking light.
Ruling Out Obstruction
For this, you will need to make a pinhole from a piece of white paper, and then look at the diamond through the tiny hole you have made. If you can still see the dark spots, that means they are cause by obstruction.
Ruling Out Prong Reflection
One of the possibilities is that two dark spots are reflecting the prongs. The only way to really tell if this is the case is through the use of a loupe or an ASET. With the former, you simply place a finger beside the prong and see if the color of your skin is visible.