How do you know if the electrical system is operating correctly?

To determine if the electrical system in a Cessna 172 is functioning properly, one key indicator is the status of the alternator warning light. When this light is off, it signifies that the alternator is operating correctly and is successfully charging the battery. An illuminated alternator warning light typically suggests that there is a problem with the alternator's ability to produce electrical power, which can lead to various systems in the aircraft losing power and potentially failing. Thus, the absence of the warning light is critical for confirming that the electrical system is functioning as intended.

Regarding the other options, a full fuel gauge simply indicates an adequate supply of fuel but does not provide any information about the electrical system. An oil pressure gauge reading zero would indicate a serious issue with the engine's oil pressure, which is unrelated to electrical system status. Lastly, an illuminated stall warning light serves as a reminder about the potential for a stall condition during flight, but it does not reflect the health of the electrical system either. Therefore, monitoring the alternator warning light is the most effective method to evaluate the electrical system's operation.