find something in the room that is 20ft away...put your fingers together and make a tiny hole or "peep sight" that you will look through at this object...now close one of your eyes...if the object stays where it is, this is your dominant eye....if the object moves, your other eye is the dominant eye...
now get a compound bow
I know most wont have one but when you aim with a compound bow, there is a "peepsight" with optics in the string through which you look and there is a main target scope on the riser (body) of the bow also with optics...when you draw a compound bow its kind of like looking through a monocular/telescope with optics on the front and back...you focus on the scope that is on your riser when you shoot at something BUT you have to keep both eyes open to have proper depth of field...only one eye will work for this...the other will refuse to cooperate...
I had to change my eye dominance because my right shoulder is beyond repair so I had to change everything around...it is an incredible mind fuck to make your brain focus with both eyes open...but you can rewire your brain to use the other eye and aim with the other hand...