Scientific way to prove you are ambidextrous, right handed or left handed?
I don't know whether the following has been used to distinguish natural from "corrected" right handers, but following up on the suggestion from @user3169, the Edinburgh Handedness Inventory contains a range of handedness tasks, many of which would probably not be as explicitly trained as writing or eating, and which thus potentially could be used in distinguishing natural from corrected right handers. 

There is an online version of the inventory here.