Fan clutch.

Hello there Chrysler gearheads .Here is my question how does one know when your fan clutch is not functioning properly. When the car is ideling the blade spins fine so it appears. But on the highway one does not know if it is ingauging properly. Is there a surefire way to determine if a fan clutch is getting bad. I cleaned it up but I believe it's original so it's 45 plus years old any tips would be great..thanks guys. It's a 440 71 New Yorker with the 7 wing/blade fan.
Author: big block