The viscous clutch keys off radiator exit air and this seems to correlate well with a coolant temp of about 100C in warm weather. Once the fan clutch tightens I can hear distinctive "fan roar" begining at about 2000 revs.

This is my "test" for a properly functioning fan clutch. Get it hot enough (100C or a little over) to engage the viscous clutch, but not hot enough (about 105C) to engage the electric fan(s). Then listen for fan roar at about 2000 or above. If you hear it, the clutch is probably okay. If not, the viscous fan clutch is probably dead.

Duke