The aux fan should come on low speed when a/c is on as long as ambient temps are high.
A quick test for fan circuit is to jumper the high pressure sw at the drier [ the one with the pigtail wires].. Just peel back the sleeve at the white connectors and jumper there w/key ON..that will give you Low fan..High fan test is to disconnect/unplug the engine coolant temp sensor [ 2 wire/blu one-105C.], That will default the ACC unit to High fan.
If both these test work, you have verified fan circuits integrity, BUT..if you never see a Low fan activation and it has passed the jumper test, then you can be slightly low on refrigerant..[ low ref. will not produce s high enough high side pressure to trigger the sw ] Very common mis-diagnosis...this condition will be coupled with a complaint of poor a/c output at stop/idle
If either of the fan speeds fail these test, the first place to look is at the fuses.