2007 E350 Check Engine Light

I have a 2007 Mercedes E350 and the 'Check Engine Light' is on. My question is if there is a way to check for problem codes without using a code scanner. Last time I brought my car in, it looked like the service manager was able to access the problem codes by pressing a combination of buttons in the car and looking at the code on the instrument cluster display screen.

My car seems to run just fine and, since I'm 90 miles from the nearest service center, I don't want to waste the time making a special trip if this is nothing more than a gas cap sensor.