Tough question. First they give almost no problems. This makes deciding extra hard. The force exerted probably can be qualitatively determined by an experienced tech, but even then its a lot like deciding about shocks.
The only feature I know to test is the thermostatic engagement. If you hold the engine at 2000rpm from about 160deg temp till it gets to stable temp you will notice a distinct change in the noise level of the fan when the t-stat closes (in the fan clutch). For this to mean anything you have to know the temp of the radiator. Most shops now use infrared thermometers to make these judgements.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician