Clutch fans are simple to diagnose. Do all hand checks with the engine off. Some folks will test by grabbing the blades while the engine is running, but that's an easy way to lose some fingers. And, just because it's never happened before doesn't mean it won't happen next time due to how clutch fans operate.
Cold - should be hand spinnable, but with resistance. This is because it has a viscous fluid inside for the locking mechanism to work and that fluid will settle when the engine hasn't run for several hours. Engine running, should be no discernable fan noise, maybe a light roar and generate a light breeze.
Run the engine a minute but still cold - should be hand spinnable, and easier to spin. The viscous fluid will have been spun up inside the clutch fan, so there's no longer a big glob of it settled to one side. Engine running, should be no discernable fan noise, maybe a light roar and generate a light breeze.
Operating temperature - may be hand spinnable, but should be difficult to turn. Engine running, should be a little fan noise, roar should be a little louder, with a little more of a breeze. The viscous fluid has absorbed heat and is expanding, which is starting to lock the blades to the shaft.
Hotter than operating temperature - Not spinnable by hand. Engine running, should be a very loud roar. There should be a strong breeze, and you should feel nervous about sticking a hand in. The viscous fluid should have the blades solidly locked to the shaft at this point.
