As far as checking the chain, you pretty much just have to go by mileage. If the car has over 100k on it I would replace it and the upper guides and tensioner for preventative maintenance. I know I will get a lot of disagreement on that, but I have seen many of these engines jump time and bend the valves. As far as checking for a broken guide, pull the left (drivers side ) valve cover and look at the inboard guide rail very carefully. It will probably be broken where the bottom pin goes through it. It will be hard to see, but it can be seen.
Mercedes Benz Master Tech
ASE Master Technician
Owner of "Behind the Star Inc."