Just ground the fan switch...take the wire off the thermal switch at the bottom of your rad and touch it to a ground if it's a Gen I bike and see if the fan works...if it does it's the thermal switch which is at fault.
The FAN RELAY is in series with the fan motor; as well as the FAN FUSE.
Thus, even if the thremal switch lead is grounded, the fan will not operate unless a serviceable fan relay and a conducting fan fuse are installed. If the fan doesn't come on when the thermal switch relay is grounded, that's not a sure sign of a defective thermal switch.
The deficiency could be the result of a blown fan fuse, a defective fan relay, or . . . a defective fan motor or any associated connection.
For the fan to operate, the device must receive power through the FAN RELAY and the FAN FUSE, whether the thermal switch is operable or not.
(A waste of keystrokes, perhaps, but . . . the thermal switch provides a ground to the control solenoid of the fan relay, closing the fan power contacts when activated.)
Besides my own personal, hands-on experience working with cooling fan circuitry, I shall now cite famous authorities bolstering my premise: Mr. Ohm, and Mr. Kirchoff.