When testing a cooling fan switch, first test it with the engine cold, then test it when the engine reaches a specified temperature. Check your vehicle's service manual for more directions.
To check a switch, disconnect the connector at the switch. with an ohmmeter, check for continuity between the terminals of the switch with the switch moved to the on position and to the off position.
The continuity through a switch can be checked with an ohmmeter. With the switch open, there should be infinite resistance. With the switch closed, there should be zero resistance.
With the switch off, there should be no continuity between the terminals. With the switch on, there should be good continuity between the terminals. If the switch is activated by something mechanical and doesn't complete the circuit when it should, check the adjustment of the switch. (Some switches are not adjustable.) If the adjustment is correct, replace the switch. Another way to check a switch is to simply bypass it with a jumper wire. If the component works when the switch is jumped, a bad switch is indicated.
Voltage drop across switches should also be checked. Ideally, when the switch is closed there should be no voltage drop. Any voltage drop indicates resistance and the switch should be replaced.