If you want to, prior to doing the idling test to see when the fan comes on, you might want to make sure the fan CAN run.

Remove the side grill (the triangular piece) that covers the riders right side of the radiator. You'll see a housing connected to rad hoses with two sensors. The sensor with the Green/Red wire (round connector) is for the temp gauge. Do not ground this wire as it can burn out your temp gauge! Next to this sensor you'll see a sensor with a flat connector and a Blue/ Green wire. This is the sensor which grounds the fan relay which sends power to the fan. Unhook this Blue/ Green wire and ground it........with the ignition on, the fan should run.....if it does, reconnect the wire and proceed with the idling test to see when the fan starts. As previously mentioned the bike will typically be right on the edge entering the red zone before the fan starts.....which it should do and prevent boilover.

If the fan didn't start doing the initial grounding test, I'd check the fan relay. On the 83 I had, the fan motor relay failed and caused it to boil over.

The fan motor relay is located by removing the windshield and the "hood" that covers the back of the instrument panel. You'll see a row of relays, the fan motor relay is second from left as you face the headlight. It will have 4 wires, 2 red/white, one Blue, one Blue / Green.

Update for testing the two wire switch:

Took a look at parts lists today.....heres what I found.
83-85 uses a thermoswitch with a single wire, spade connector.

86-93 uses a thermoswitch with 2 wire, spade connector.

So I removed the right "side" grill piece from my 86. The thermoswitch points down and has 2 wires, a Blue and a Brown.
Unplugged connector, with bike cold the thermoswitch is open (no continuity). Turned on key....you should have battery voltage on the Brown wire. So I ran jumper between Brown and Blue wires, fan started.
Unhooked jumper....hooked test leads to thermoswitch and monitored continuity with engine idling...switch "closed" about 1/4 inch from top of green on temp gauge, so I hooked connector back up to start fan.
So...if jumpering the connectors starts the fan, fuse and fan are fine...thermoswitch suspect.
If jumpering does not start fan, check the fuse. I've only had the top cover off my bike a few hundred times....NEVER recalled seeing the 2nd fuse box!!!! Its directly in front of the battery. It says FUSE on it. Remove the cover and there are 4 fuses (spade type).

Audio
Backup (memory for Audio)
Fan
Hazard Flashers

Hope this helps!