I just had my A/C sensor (incorrectly) diagnosed as bad by a Mercedes dealer. The part cost is not bad, it's the labor that's nasty -- the compressor must be removed. Labor to replace the sensor was about $300.
My problem eventually turned out to be the MAS control unit in my '92 300E, which was especially frustrating since I suggested that as a potential problem (I was having a fuel-pump problem, and the MAS unit in my car is in place of both the fuel-pump relay and the KLIMA relay used for the A/C).
Anyway, the dealer that correctly diagnosed it said that the compressor can be spun manually and a reading taken from the speed sensor while the compressor is moving.
If you decide you need the speed sensor, I've got a used one for sale cheap.