The whole point of the ending is that Cobb does not care about whether he is in reality or not, as long as he is with his children.
To search for clues as to whether it's one or the other is to miss the point.
To search for clues as to whether it's one or the other is to miss the point.
Whether Cobb cares or not he's still somewhere.
Or you can look at it that there are two endings depending on what you're looking for - certainty or something other.
Using the ring as an indicator allows someone to nit pick that he may be wearing the ring in that scene and that something happened between the airport and the car ride home.
Using the fact that he didn't care to verify solidifies the real result... he's given up fighting.