An oxygen sensor kit is the easiest for rich, normal, lean indication but only works with unleaded gas. They simply read off rich, lean, or on.

The absolute temperature varies by engine, where the EGT sensors are placed and other factors.

On airplane engines, EGT is used by adjusting the mixture control to get the highest temperature, which is the optimum burning air-fuel ratio. Then the pilot either turns it richer, until say there is a 100 degree temperature drop, or some pilots run them leaner. The temperature drops each way off the peak (rich or lean). Lean of peak gives best fuel economy, but could burn the pistons, valves, etc if not careful. Rich of peak is safest setting for the engine. Slightly rich gives best power. Richer still cools the engine, especially under turbo boost.

For emissions controls and fuel economy, most auto engines (carb or fuel injected) run just slightly lean of peak, except at full throttle, when they run (hopefully) at the max-power rich setting.