The quick and easy test for a viscous problem is to (carefully) stick a fresh carrot into the fan blades (engine running up to temp of course)

If the carrot stops / stalls / significantly slows the fan, the viscous coupling is gone.

Nothing actually stopping you botching a viscous coupling to make it locked up permanently, it's only a fuel saving measure, a fan will draw up to around 1.5 BHP at revs.