you go look on the front of the motor, on the passenger side front there are two sensors in the coolant passage, you can use a multimeter to check their resistance when the engine is hot and cold, to verify that the sensor is working correctly. i don't have the resistance values off hand but you can search for them.
with the two wire sensor you just measure across the two contacts, with the single wire sensor you reference anywhere metal on the engine for ground.
with the single wire sensor (that goes to the gauge cluster) there is a good chance that something is wrong with the gauge cluster though, which is more work. start by checking the sensor and the plug going from it first.
1992 SC400 black, 1JZ VVTI with 350Z 6spd swap, running!