Took the car out yesterday, slowly heated up to about 240 degrees and I took her home. With everything warmed up, I shouldn't be able to spin the fan easily by hand, should I? It's a thermal clutch.

 

The clutch is supposed to get stiffer but not completely lock up when hot air comes through the rad.

 

How do you tell if it is bad, then? Try a new one and go? Like I said, it spun pretty easy by hand.

 

Maybe time to convert to an electric fan. It'll free up a few HP as well.
It was easy to tel when the one in my 84 Z28 went bad - it was leaking it's fluid from the seal around the shaft.

 

You could try popping the outer end of the bimetal spring out of the clip and check how much stiffer the clutch gets when you twist the center shaft to its limit in either direction. I don't know if that would actually be useful or not. Usually if there's any question if it's good or not it needs to be replaced.