For the rest of my trip, I don't think to look. I only think about it when I'm driving and it's time to fill up the tank. I try to use the side view mirrors to see which side the gas cap is on. It never works. Then I look at other cars on the road, hoping to spot one that is the same model as mine so I can see which side the gas cap is on. Unfortunately, I'm car blind in the way that some people are face blind, so that usually doesn't work. I usually end up driving to the pump and finding out if I got lucky.
But last month I met Joshua Schacter (creator of Delicious) at TED and he told me that most late-model cars have a little arrow on the gas gauge that points to the side of the car with the gas cap. This information has changed my life.
Joshua's latest creation is ClueDB, a website "for sharing tips and tricks on how to make life better. You can tag, vote and comment on the clues you like. And you can contribute clues as well." Sample clue: "Putting a screw into a threaded hole -- Turn the screw backwards until it clicks into the first thread. the chances of putting it in wrong are much less that way."