I gotta admit, as a right handed/right eye shooter, this technique never crossed my mind. It is so simple that I'm sure figuring out if she is left eye or right eye dominate will be more difficult.
I would ask her to wink at you a few times, most people will leave their dominate eye open. This is what i ask new shooters to do after i ask them if they are left or right handed.
My $0.02.
Lefty.