The Forgotten On

All you need to do is spin the fan on the pulley. If there is no resistance, the clutch is toast. If you have some resistance it is good to go.

No need to get it hot.

dr bob

I think a major killer of the viscous fan couplings is taking them off and storing them with the drive vertical. The fluid leaks out that way. If you have one out, store it in the same attitude it was in while mounted, with the drive horizontal. WSM has a note about this somewhere.