here is the easiest way:
To Find Your Dominant Eye
This experiment is called the Dolman method of determining ocular dominance. It is quick and easy.
Take any piece of paper or cardboard and, somewhere in its center, cut out a square or circle. Make it about one inch wide. It does not have to be a precise shape.
Hold the paper with both hands at arm’s length in front of you.
Look through the hole at any object nearby that is not moving – let’s say it is a desk chair.
As you continue focusing on that chair, gradually draw the paper closer to you until it is touching your face.
You will find that the hole is in front of one of your eyes – your dominant eye. That eye has been the one focusing on the chair all along, while the other eye looked at it a little from the side.