Simple Low Fan test .

Jumper pigtail wires going to drier/reciever high pressure switch.. if fan runs , you have verified the fans electricals [ motor/relay/fuse/resistor/etc]
This indicates you do not have enough high side pressure to trigger the fan high pressure switch.. most common probable would be low on refrigerant.
There is also the remote possible of having a bad switch, but very few fail this test.
Also remember , as LD posted, that the fan will not come on until it is called for .. meaning the system/switch has detected a HIGH THERMAL LOAD CONDITION... usually due to high ambient temp/humidity. The aux fan is only designed to relieve the high pressure when the system is being TAXED,, in cooler ambient temp, it is quite common to have stisfactory a/c output without the high side needing aux fan inputs b/c the high side does not get to the sw cut-in spec and is therefor unneeded.
A truer test is to use gauges for switch spec comparisons, but those who know these systems well can test fan operation in 30 seconds with a simple jumper wire and observation of the high side Eye-Sight Glass..