That military does not have or issue a left handed M16, but they do have a lot of left handed shooters. That's why the brass deflector is there.
The military did not have left handed Garands. They forced you to shoot right handed. Millions of GIs did with good results.
To check for your dominate eye.
Look at an object. Both eyes open of course.
Open your hands so that they are flat, fingers together, thumbs outstrectched.
Put your two hands together, one over the other, so that you have a triangular opening, The sides of the triangle are your index fingers, and the bottom is your thumbs.
Hold your arms straight out in front of you and look through the triangle. Look at the object you picked earlier.
Slowly draw your hands back, maintaing focus on the object, until your hands reach your face. Do not loose sight of the object. When your hands reach your face you will be looking through the hole with your dominant eye.
Don't cheat.
You can also have a friend help. Stand a short distance away and point at your friend. Stand sqare to him not off to an angle. He will be able to tell, at a distance, which eye lines up with your finger.
Most men refuse to switch sides or even try. Its not as hard as you think. And with practice the results will please you.
If you just can't, or won't do it, then mount an Aimpoint or Eotech and shoot with both eyes open. Mounting it further forward will help too. If you are left eye dominant and shoot from the right shoulder you will naturally roll the rifle to accomodate your dominate eye. Using a red dot sight, and mounting it forward will mean less rolling of the gun.