With your glasses on, find a distant object such as a tree top or chimney to focus on.
Form a triangle by putting your two thumb tips together and your two first fingers together.
Raise your arms and look at the object through the triangle with both eyes.
Then look with one eye, then the other.
You will see something different out of both eyes, the eye that sees the object properly in the triangle is the dominant eye.
This demonstrates that you don't use both eyes for everything all the time.