The quick way to test this out is to simply whip out a Power Probe. You will need to take into account the wiring of the vehicle (i.e., is the fan grounded/powered seperately from the PCM?). Depending upon how the fan is wired you can check to see if the fan is receiving 12V and even jump the circuit. In this way, you can prove if the fan alone is bad or if the PCM or wiring is at fault.
Most fans are grounded by the PCM, so with the Power Probe you can bypass that ground and make your own. If the fan spins, you have a problem on the ground side. Sometimes, the fans blow fuses when they have high internal resistance. You can check amperage on the fan with an amp clamp or a Power Probe Hook when performing this test. If fan amperage is too “close” to the fuse’s amperage rating, the radiator fan should be replaced.
If you see a good degree of “Dex Mud” in the radiator, you might have a blockage. To diagnose a clogged radiator, the simple way is to pull its hoses, let whatever residual antifreeze there is drain out and then blow shop air through the radiator. If you cannot successfully flush out the radiator, replace it.
One of the rarer problems is a defective water pump impeller. A simple way to catch this issue is to pull a hose where the water pump is and see what kind of force the pump shoots the antifreeze out at. If you are confused about which hoses are at the inlet and outlet sides, get some clear plastic hose at a home improvement store and put those in place of the other hoses. Then, turn the car on to monitor coolant flow. This trick can be used to monitor flow in and out of the heater core as well.
Lastly, thermal imaging can assist in doing many of the preceding diagnostics. It can quickly discern the temperature of antifreeze at different points of the cooling system. The most advanced thermal imagers can look into a radiator and see where the flow of hot antifreeze stops. However, sometimes there are so many hot parts on a vehicle surrounded by other hot parts, it’s not possible to get that precise. But, an array of diagnostic equipment can help the technician nail any cooling system issue.