There are several parts to this as I see it. You need to measure or sense the temperature then make a comparison of the two temperatures.
There are numerous sensor types. Among them are thermistors, RTDs, thermocouples. A thermistor is probably lowest in cost however you'll have to figure out how you'll get it to "see" the temperature of the fluid you'll be measuring. Clamping it to a metal pipe (without crushing it) might suffice.
The thermistor changes resistance with temperature - increasing or decreasing depending on the coefficient. With the proper electronics, which could be as simple as some resistors, you can end up with a voltage that varies with the temperature. You would need to do this for each temperature that you are measuring. I do think that the current flowing thru a thermistor should be very low though I have no advice to offer on how much.
You could then compare the two voltages with a comparator. An LM339 is common and low cost. I understand that an op amp, like a 741, can be configured to perform that function. The output of the comparator will either be low or high depending on the difference between the values. You might want to add some hysteresis (see applications literature) to keep the circuit from toggling.
What I've done is described to parts - the temperature sensing portion and the comparator portion. I'd seach for info on each piece - or maybe others can provide info - then essentially put them together if someone hasn't already done that and made the information available on the web.