I came across this blog post today about a easy way to know what side your gas goes in. I have never noticed it before like Steve that all you have to do is look down at your gas gauge. Most cars will have a arrow pointing in the direction where you fill up your tank.
Another way I was told today is if you do not have the arrow the gas gauge will be to the left or right of steering wheel indicating which side you fill up.
The arrow is right above the E in the picture below next to the gas pump icon. This picture is from my wife’s car and it is on the left side as the arrow indicates.