I don't know about the rest you, but I have been just a bit confused - until now - about this thing of testing fan clutches. So I would like to share my enlightenment directly from the MBZ Maintenance Library.

For the Visco Clutch (the type most of our MBZs use):
"Checking cut-in temp - Run engine at 4000 to 4500 rpm. When coolant water is at 90 - 95c, the fan speed should increase by approximately 1000 rpm, which can be checked clearly acoustically." This is for the M110 engine, so different engines may have slightly different temps, but the procedure is the same.

The magnetic clutch is the one you check by turning the fan by hand - w/engine OFF - and noting a slight resistance.

Another thought: Have you ever noticed how much crud accumulates between the cooling fins on these visco clutches? By keeping them clean, I would think the oil inside would flow from one chamber to the other at the proper temp. So next time you have an appearent fan clutch problem, check the amount of crud on the cooling fins as well.

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