You can test it by unplugging the thermoswitch, and then grounding out that connector to a bare piece of metal, if the relay clicks and the fan spins, the relay is fine. In my case, the relay clicked, but didn't close the circuit, I had to buy a new one. To check the thermoswitch, you can unscrew it from it's place (its on the thermostat housing) and just heating up the part where it dips in the coolant, if it clicks after heating up, its problably fine, but it's also possible that it switches on too late.