Fan clutch test:

Both the S4 & S5 clutch freewheel when cold.

The s4 clutch increases it's bite gradually with temperature rise.

The s5 clutch is more on-off. (It just coasts until is gets warm)

Once they are warm, the fan speed goes up with engine speed to about 4000 RPM.

As the engine revs higher the fan stays at ~4K.

If your warmed up fan only revs up to < 2500 RPM, it's worn out.