There are two ways to do this.
1. Extend your arms straight out and form a triangle with your hands. Looking through the triangle with both eyes open, frame something nearby (like a doorknob) and place it in the centre of the triangle. Then close your eyes one at a time without moving the triangle — your dominant eye is the one that placed the object in the centre. The eye which doesn't see the object in the centre is not your dominant eye.
2. Make a hole with your hands while your arms are stretched in front of you. Place an object in the middle of the hole and now move your hands slowly towards your face. Soon you will find that the hole you made is in front of your dominant eye.
via reddit and petapixel