Toyota Repair: 1991 Corolla overheating, ohm meter, looseness

Question
I have a '91 Corolla with a 16 valve 4 cyl. engine with an automatic trans, and 140000 miles on it.  It will not drive very long without overheating. I have checked for leakage/looseness on the water pump with no luck, and can watch plenty of water moving thru the radiator. I have checked the thermostat and it appears to be working smooth and looks new. The only time the fans come on is when it get to the red on the gage and is blowing steam. How do I check the fan switches and relays to decide which one to replace?

Answer
Since the fan does come on it's not a relay problem, it may be a problem with the temp control switch, it's located on the thermostat housing,try disconnecting the switch and if the fan runs all the time the circuits and relays are ok, drive it this way for a while and see if it still overheats. The switch can be tested using an ohm meter, there should be continuity(closed) below
180 degrees and no continuity (open) above 199 degreesF.