Most temp gauges work on a thermal resistence principle,i.e.the higher the temperature the lower the resistance thus a higher voltage is passed to the gauge causing the pointer to register a higher temperature.
So one terminal is connected to the supply,12v, and the other is connected to the gauge in its simplest form.Is there a reading from terminal one to terminal two???😎😎.
If the sender,sensor,has only one terminal it is a thermoswitch,which on reaching the excess temperature will short out,i.e.have a zero resistence,thus completing the circuit and switching the warning light on.
A reading of between 150 to 450 ohms appears to be the most common value,both for switches and sensors however this is not set in stone