I think Nolan didn't leave a final answer because Cobb has to live with this question for the rest of his life. His totem, which is supposed to never be handled by anyone else, came from Mal. Additionally, we dont know when he received this, so if it is while he was dreaming or in Limbo, it may be a false totem. Sadly, i am at the point where Nolan saw Cobb's emotional arc more important then specifity in the plot. Cobb may not care if he is dreaming, because he made it back to his kids.
It is a minor copout, but I hope when I watch it again, I can a piece of evidence throughout... I have heard his wedding band correlates to dream state and reality.