My preheat indicator is the engine dipstick.
If you pull out the dipstick and you can get a drop of oil to drip off of it, then the oil is will flow for start up. If the dipstick looks like a popcicle on a dipstick, and oil will not drip off, then you will need to preheat.
When you preheat in the usual fashion using external heaters, put them at the cowl flap area not in the intakes. Remember, heat rises. Block the air intakes so the heat will not escape and put a blanket over the cowl if you can.
Preheat usually takes a while.. Remember, if you reach in and the feel the engine, and it feels warm, that is not the crankshaft or the cam that you are feeling. It will take a few hours for the heat to reach them.
Electric heaters such as Tanus that are permenately installed are very good. Dip stick heaters tend to just heat the oil around the dipstick, and not the whole engine.
Turbo charged engines with their external plumbing to the turbo shaft bearings and the waste gate are another story. They should be preheated if the temperature is near or below freezing to prevent congealed oil in the external lines. Congealed oil in the right place in the waste gate line can mean you will be in for a big surprise on start up.
An airplane in a closed hangar can usually be much warmer than the outside overnight air temperature.
Bill