1990 Benz 300E 2.6 (mine) ; 1994 E320 Wagon (wife's) ; 1993 300CE Cabrio, needs some TLC
Can one see the condition of the tensioner rubber when it's on the car? I noticed that my M103 belt is rather new but not sure about the condition of the tensioner..
no, youo can't. its quite buried, and you really wont know its condition til you release the tension via the golden hollow tube-nut, and remove the whole assembly.
see the flats on the shaft? draw an imaginary line through the center of that shaft, parallel with those flats. if that points to the pulley, like the one on the left, its good. if it points off to the side like the one on the right, its toast.
VERY late model M104.99x have an entirely different spring loaded tensioner that doesn't have the piston damper, or the adjustment bolt, these last forever and are cheap. catch-22, you need to replace the power steering or tandem pump mount in order to switch to the new style.