One failure mode I've noticed is oily wet dirt/dust stuck to the clutch fins. I.E the viscous fluid is leaking out and failure maybe in the near future. Mine was like this when I changed my belts earlier this year, but still functioning at the time. Something to look for. My 540i fan clutch looked the same when it actually failed. (More noticeable on the M62 motor due to the higher operating temp)
I just did rad replacement on my 2001 M5 and the fan clutch on it was clean even though the gap between the rad & condenser was chock full o'crud. (No oil leak on clutch). My 2002 M5 had a wet spot radiating from the center of the clutch and oily dirt caked up on it.
Was the dirt oily/wet looking? Did it look wet from the center outward?
If it stops moving quickly after you let go probably still OK but if the oil is leaking out maybe on borrowed time.