Scenario: You've borrowed someone's car, and you have the courtesy to fill up their tank. You pull up to the station and "Hmm, which side is the cap on?"
Solution 1: Assume it's on the left/right, pull up to the pump, get out, swear under your breath, move car to a different pump with cap facing the right direction.
Solution 2: Find the gas gauge on the dash. Find the arrow / triangle. Whichever direction the arrow / triangle points is the side of the gas cap.
Genius, isn't it? I learnt this about a year ago. It's nice not having to do guess work in a rental when it's -40 out.
Mine's on the right. Where's yours? :P