These things are realy expensive (i think someone mentioned 500+$) Use a VL one or use an electric fan - much, much cheaper
I actually have a brand new one from a vl with a few klms on it if your interested
You'll need a mate to test, but once the viscous fan gets to a certain speed it shouldnt go any faster, just keep revving the motor up high and see if it increases with rpm all the way up the scale. If anything it may of shift indicating wear
You can also just turn the fan with the car off and see if its fully seized