If you were the person who checked first before you're driving you would not need to go through all the hassle, but if you did not do so it isn't a hassle as most modern cars nowadays make it easy for you to tell these days. All you have to do is take a look at the fuel gauge at the car's instrument cluster. If you look at the photo above you can clearly see a small triangle-like arrow right beside the icon that looks like a fuel pump. Take a look at the photo below.
This little arrow is used by most manufacturers these days. The photograph in the paragraph above is from a Mercedes Benz AMG C63's instrument cluster and it uses this simple icon to show where the filler cap is situated. The photos below is from a a Suzuki Swift and from a Corvette.
Some cars like the 2005 Impreza makes it even clearer. It states 'Fuel Door' right beside the little arrowhead (photo below). It does make life a little easier for those that actually drive a lot of cars all the time.
But it isn't a full proof tip. Most older cars don't have this little arrow and some newer cars also don't have it. The 2010 BMW 5 series Gran Turismo (instrument cluster below) is a car that doesn't help its driver (or the person who stole it) any indication as to where the fuel filler cap is located. That is unless the driver stepped out of the car to make sure before he or she stopped at the pumps. It's either that or he'd better make sure the fuel hose is long enough to cross the width of the car.