Very simple! With both eyes open, point your finger at something like a car, rock, whatever, that's about 50 feet away. Close one eye. If your finger appears to move away from the object, you have closed your dominant eye. Check it several times. If it works equally with either eye, you have no dominant eye. If, however, you obtain the same result time after time, you have a strong eye dominance. How to correct for that if your dominant eye is not the one behind the gun is a whole topic unto itself.