Good opportunity for a discussion.
The electric fan is there to get cooling airflow through the radiator core when the incident airflow isn't enough, like when in traffic or stopped. A reliable measure of how well the radiator is coping is the temperature difference between the top inlet and the bottom outlet.("delta T") If we have a thermostat, we can know the water inlet temp. The closer the outlet temp gets to the inlet, the less cooling effect the radiator is having.
A good electric fan and shroud can really get a cooling air blast through the core, and it works quickly.
I have always used the outlet temp from the radiator as the process switch for the electric fan. While this is the logical place to sense the temperature, it is not popular because the setpoints require some experementation to establish inasmuch as every installation is different as far as temperature drop across the radiator goes. Here, our hobby shop equivalent of Radio Shack sells an adjustable temperature switch, and with an engine tstat of 180F I usually start by setting it to switch "on" at say 160degF , and "off" at 150degF. If the fan comes on in normal driving this suggests the cooling system is perhaps struggling for some reason that requires investigation. OR taking the "on" switch point up to 165F may fix it. (theory tells us, the hotter the water in the radiator, the greater the heat loss for a given atmospheric temperature)
Other traditional positions for the fan switch, like at the thermostat housing or in a cylinder head certainly work, but operate when an overheat condition already exists rather than intervening to prevent it. IMHO.