Extend both arms and make a small triangle with your thumbs and index fingers
Choose a distant object and center it in the triangle
Keep both eyes open and focus on the object
Close one eye
If the object stays centered, the open eye is dominant
If the object shifts out of the triangle, switch eyes
The eye that keeps the object centered is the dominant eye
Another method is to point at a distant object with one finger
Close one eye, then the other
The eye that keeps your finger aligned with the object is dominant