What part of a fire extinguisher indicates that it has been used?

The part of a fire extinguisher that indicates it has been used is the pressure gauge. When a fire extinguisher is deployed, the pressure inside the cylinder is released to expel the extinguishing agent. The pressure gauge provides a visual indication of whether the extinguisher is still pressurized and ready for use. If the gauge needle has dropped into the red zone, it indicates that the extinguisher has been discharged or is no longer functional.

The discharge nozzle, while essential for directing the extinguishing agent, does not show evidence of whether the extinguisher has been used. The handle is important for operating the extinguisher but does not reflect usage status. Lastly, the inspection tag is utilized to track maintenance and inspections, but it does not provide direct evidence of whether the extinguisher has been discharged. Thus, the pressure gauge is the key indicator of whether the extinguisher has been employed in an emergency situation.