E Pluribus Unum

Pssst -- it's on the gas gauge...

More on this...

Regarding the gas gage, the tank if often on the same side of the car as the gage is of the instrument cluster. So if you look at your gas gage and it is on the right side of the speedometer, your gas tank is almost ceratinly on the rioght side of the car. If it isn't, it almost always has the arrow....

Foreign cars typically have the gas tank on the driver's side - US cars on the passenger's side. (This isn't a 100% rule, but its its pretty common).
Next time you get off the Jersey turnpike (or similar freeway) and there's a lot of long lines for gas, you might notice that for the most part American-made cars are in the long lines, while foreign (and more often European than Japanese/Korean) don't have to wait that long because the gas tank is on the opposite side....