There is a thermal bi-metal spring on the front of the clutch. When the radiator gets really hot, that spring moves and closes an orifice inside the clutch. Then the fan clutch should 'lock up'.
When I was in doubt of a fan clutch back when I was a tech at the Toyota dealer in the 80's, I would get the car good and warmed up, then slip a piece of cardboard in front of the a/c condenser (all of it). Leave the hood open. Then I'd sit in the car and hold it at 2000 rpm and watch the temp gage rise.
The clutch should lock up before it gets to the red zone. When it locks, you'll here a hellacious roar, cause it will be pulling a massive amount of air. Note the temp it comes on at, then let it idle and get out and remove the cardboard. Let it idle till it cools back down to normal. The clutch will release somewhere above 1/2 on the gage.
If it goes up to the edge of the red zone without locking, I'd replace the clutch. If the fan blade does not spin when engine is off, replace clutch. If the blade spins around more than 1-2 turns when engine off, replace clutch. approx $80