For what its worth, I keep my block heater on a timer so that it's on for ~3hrs before I start the car.

The coolant temp gauge just barely registers that the car is warm (at about 40 degrees celcius). You can also reach under the car and feel the oil pan, if its not freezing cold (37 degrees centigrade is body temperature) then the heater is working.

While I haven't yet had any problems starting the car in cold weather without the block heater, it definitely makes a noticable difference on those frosty mornings.

1985 300TD-T