My estimated probability is extremely low because unlike the patient with anosognosia, I observe movement in my left arm. The patient did not observe motion and invoked absolute denial in inventing explanations for it.

Still, I'm not a diagnostician so I'll assume patients with anosognosia think they can actually observe motion in their left arms. In this case, it seems the popular proof is in doing something that requires both arms, like JulianMorrison did. Still, it seems that if you can deceive yourself about your arm moving, you can deceive yourself about such an experiment. It strikes me that one way to prove your left arm still works is to disable your right arm. After all, you have no illusions about your right side - if you disable your right arm, you'll know it. Therefore, I shall tape my right arm to the arm of the chair I'm sitting in, and finish typing this post with my left. There. It is currently 10:52 EST. The post timestamp will agree that this post was submitted before I released my right arm.