I was trying to decide how to deduce whether or not I would know when I've completed this goal, and then realized that every time I check in with my Wii it keeps track of your center of balance (COB) - which is a direct example of your posture! And here are my results:
Now... you see how my COB says it's slightly to the right. Why that is you might ask? Because I have one plate and six screws in my right ankle making me FOREVER, slightly to the right.
To even further affirm my new and improved posture... I looked to my yoga poses which have shown incredible improvements on every single one - all showing 90pts or above for fabulous COB.