If you did not personally adjust them last time, you have no way to answer your question short of tearing the engine apart. If you are concerned, recheck that valve in 5,000 miles or so.
Was that particular valve hard to get to. As I recall, the valve that is second from the back of the motor is difficult to get to. If it is this one, someone probably was just too lazy to mess with it.
Best of luck and Happy Thanksgiving,
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in