As Steve says, rarely ever the switch, but gages are required to see if the a/c gets to the trip pressure spec of the switch.
If only a couple of ounces low on F-12 charge, this will create that condition. [ cooling, but not enought high side pressure to trip fan]
A point to note is that if you are running at low thermal load
[ low ambient temp], it is possible for the fan not to be called for .
This is something to consider in Northern climates, where one will not see that condition exist in Hot climates [ where high thermal load is constant]..
If you jumper the switch and fan works , then all electrical components [ except possible switch] are OK, including the dropping resistor..
With R-12 , [ as compared to 134a],you can judge freon level fairly well by observing the eye-sight at the drier . If it is clear and still no fan, likely the sw, but if foamy/bubbling, likely low on freon and just not getting to sw cut-in pressure spec [most likely probable]..