My wife is right handed but left-eye dominant. I've long thought my boy was the same by the way he naturally uses his left eye to look through rifle sights but it's been difficult to instruct him to test it.
We visited the Museum of Science this Saturday and they had an area with all sort of vision experiments you could try. Now I know for sure he's cross-dominant, just like my wife.
First Google hit had a really good discussion on the topic:
http://www.northcarolinasportsman.com/details.php?id=519. It would appear that the best way is to teach a young person to shoot with left hand.
I'd love to hear from the NES folks your own experience of overcoming cross-dominance.