I don't understand. You want an indication of which side the gas tank is on ?! Wouldn't it be the side where there's a flap/cover thing that you pop open prior to unscrewing the cap and pouring the gas ?
Edit: I had to leave the office, go downstairs, get in my car and check. Whatddya know, a little arrow next to the gauge, you learn something every day.
This is most useful when you are renting or borrowing a car. If you don't check before you hop into the car, you need to physically exit the car and move it if you guessed incorrectly.
There's no real reason why you can't fill up a reasonably large car (I (in the UK) have a Ford Galaxy) on the 'wrong' side of the pumps. Sometimes you have to be a bit more careful about positioning, but most hoses are long enough.
Edit: I had to leave the office, go downstairs, get in my car and check. Whatddya know, a little arrow next to the gauge, you learn something every day.