At the eye doctor the other day, getting tested for LASICK and they way he tested me was to hold a piece of paper with a 1" hole in it at arms length with both eyes open until I had an object in the hole. Then have somebody cover your right eye and then your left eye. If the object stays in the hole with your right eye, that is the dominant eye. If the object dissapears with your right eye, but stays in the hole with your left, then the left is your dominant eye.

I would imagine that this works well even if you are just winking from eye to eye and not having anyone help you. (it does, I jut tried it )