I got it from here: the trad gang beginner article-
The very first thing to learn is which of your eyes is the dominant one - the one eye that focuses directly and clearly on an object. Hold a hand about a foot in front of your face, make the "OK" sign circle by touching thumb and forefinger, use both eyes to center a distant object within that circle, close your left eye - if the object is still within the circle, you're right eye dominant; vice versa if the left eye centers the object. If you're right handed and right eye dominant, or left handed and left eye dominant, no problem. Generally, right eye dominant people should enter shooting sports as right-handed participants. Left eye dominant people should participate as lefties. Exceptions can be made to this general rule, but most of these exceptions have to do with physical adversities.