You have to remove the cam cover to be able to see the chain. You can also see the cam "timing" mark when you remove the cam cover which kinda gives you an indication of the amount of chain stretch. Do you know how often the oil was changed in the past - the chain condition is directly related to the frequency of the oil changes. If there is any doubt - go ahead and have it changed. If you change it yourself you will spend less than $200. When I recently changed it in my 300D I replaced the tensioner at the same time. If you pay an independent to do it my guess would be between $400 and $500. If it breaks the cost will be at least 3x these amounts. Figure the MB dealer will be the most costly.