If the sight glass is foamy/lots of bubbles,
it is low on freon. Very few and almost completely clear is what you want.
The sw. test would need gages for pressure readings.
If the freon needs topping off, the fan sw. will then work as the pressure will be back up to normal.
Be aware that the fan is more likely to come on with higher ambient temps. If it is cold outside , it is possible for the fan not to be called for.
Some guys actually wire a switch in the cabin that jumpers the pressure switch and use it at their own discretion.
I used to do that to all my old Benz so I could get a jump on the heat. Also nice if in traffic/up- hills , etc. when the temp. gage starts to creep up there.