If you aren't sure what eye is dominant, hold your hands out at arms length and make a triangle with your left and right thumbs and fore fingers. Place an object across the room in the center of the triangle. Close one eye, then close the other. Whichever eye had the object centered is your dominant eye. If your dominant eye isn't the same as your dominant hand, I have a method that I used to change it:
I would practice attaining a site picture (always with a cleared weapon with NO ammunition even in the same room as me) with my dominant eye closed and slowly open it until both eyes were open. As I was opening my eye, I would force myself to focus on the site picture I already had so that I was training my brain that was the one I wanted to focus on. It was a slow process, but after a few sessions, it finally took.