Like a normal car engine a model car engine has a specific working temperature to be watched.
At this temperature the parts like piston sleeve are expanded to their optimal working grade.
Depending on factors like manufacturer, engine type, cooling head, humidity, fuel, glowplug, settings of the carburators, muffler and outside temperature, this may affect your engine performance and so the right working temperature.

The right working temperature is between 90° and 110° degrees Celsius measured at the right side bottom of the coolinghead and depend on all variables described above.
So this 20 degrees Celsius difference can be different for every situation.

Never run your engine lower as 90 degrees Celsius as this will wear out the parts like piston/sleeve quicker.

Temperaturs over 110 degrees Celsius will only do harm if there is not enough lubrification.
Some engines are running hotter as others but as long as the lubrification is OK then that there is no problem.
Lubrification depends on the carburator setting and fuel type used.

Therefor use any temperature device for getting a reference point to check if there is an abnormality (positive or negative) when the engine is used later.
Always first run an engine a few laps on the track and then adjust the carburator until you feel it runs OK.
This means good power and still smoke coming from the exhaust.
Then check the temperature and use this a reference point for this particular engine in this particular situation.

When using other engines perform the same as described above. Keep record of each engine specific temp.

Do not think what is a right temperature for your engine is the right one for others too.

There are different methods to measure your engines temperature.

1] Spitting
2] Fixed sensors devices
3] Infra Red handheld devices

Always first check the engine for it settings as being RICH and then lean out so it sounds OK. Then measure the temp and confirm your settings.

1] The cheapest way of measuring the engine temperature (also the one least consistent) is to put a little fluid spit (Yes from your mouth) on the engine head. 
When it stays for longer as 5 seconds the engine temp is to low. Change your carburator to a leaner setting as soon as possible.
When it stays for 3-5 seconds the engine temp is OK and around 100° Celsius.
When it evaporate in less then 3 seconds the engine temp is to hot. Change your carburator to a richer setting as soon as possible.

2] MIP and Venom Racing and some other manufactures have a self powered digital LCD temperature module that can be fitted directly in your car.

The temperature module from MIP

A loop with a temperature sensor is strapped around and put between the ribs of the engine cooling head. The modul is powered by its own battery. When the car stops in front of you, you can instantly check what the temperature is. 
The problem with off-road buggies are the heavy vibrations going through the car.
The wrapped around sensor is giving unstable readings and we advise to get a fixed mounting sensor to the right side of the engine at the bottom of the cooling head or fixed with one of the coolinghead screws. Look here how todo this

In Europe there is a brand called EOS that is also making an in-car temperature modul. Instead of a LCD this modul works with alternating LED's. The sensor must be placed between the ribs of the engines cooling head what is not giving a reliable engine temperature reading. To power this modul a connection to the receiver battery must be made.

In this case the EOS modul is build in a Kyosho MP5 receiver box.

If your are known to electronics you can make your own temperature modul system. 
In-car or wireless the choice is up to you.
Check it out at the electronic projects page.

3] Infra-red measurement is also a way to measure the engine temperature with out making contact to the engine. 
Brands like RAYTEK, OMEGA, and EXERGEN are producing these industry usable products. 
For reading temperatures with infra red devices it is advised to measure the temperature of the glowplug itself. 120-140 degrees Celsius are common readings for a good working engine. 
Click here to go to the Infra red device review section

If you like to know more about how to use Infra red devices correctly click here

Read more about engine temperature in relation with cooling head size and carburator setting!

Tip! If you just started your engine do not overload it directly, but let the engine get slowly to his working temperature like a normal car, in 5 or 6 laps. This will save your engine and expands the lifetime.