As for overheating, it can happen, it is just not likely on a beginner plane setup. I'd agree that you don't need a temp gun. However, they can be useful. If engines are running too hot, it can be a sign of over-proping or lack of ventalation that otherwise may not be detected. Beginners typically will not need to worry about this since thier engines will be mostly exposed, and if over propped, a small 2S glow (typical beginner engine) won't overheat much. A more likely cause of overheating for beginners is lean run, which is easily detectible by many methods. It is commonn to set engines on sound and smoke, which works well for many engines and fuels, but if you are weary of that method, get a tach and set needles based on RPM (essentially this is the same as the sound method, i.e. engine pitch, tone, etc. is related to RPM.) Just note that different engines and fuels smoke differently. Get to know your engine so you know what normal is so that you can easily detect when something is not normal.

As for max temp, generally engine temp should not exceed low to mid 200F. But this will vary between engines, fuel, 2C vs 4C, and even where on the engine you measuere the temp.

Cheers