Yes, I should have made a mental note when I picked the car up which side the gas cap was on. I'm pretty squarely type-A personality, but noting such a thing is too obsessive even for me. So, instead I look for a little arrow on the instrument panel next to the gas level.
For everyone that knew this beforehand and doesn't think it warrants an Instructable: I can already feel your smugness, so there's no need to comment. Let me and the one other person on the planet who didn't already know this enjoy our new found knowledge.