How can you verify if a water gauge glass is properly set on a firetube boiler?

To verify if a water gauge glass is properly set on a firetube boiler, filling the boiler to the normal operating level is essential. This process ensures that the water gauge glass accurately reflects the water level within the boiler. When the boiler is filled to the proper level, the water should be visible in the gauge glass, allowing operators to confirm that they can monitor the water level accurately.

This verification step is crucial because an improperly set gauge glass can lead to inaccurate readings, which might cause unsafe operating conditions. If the gauge glass is set too high or too low, it may not provide a true representation of the water level, which can lead to issues such as flooding or running the boiler dry.

The other choices, while they might provide some information about the boiler’s functioning, do not directly confirm the correct setting of the gauge glass. Measuring water depth with no steam provides a snapshot of water level but doesn't ensure the gauge glass is reflected correctly. Inspecting the boiler pressure gives insights into the overall operation but does not pertain to the gauge measurement. Checking the blowoff valve focuses on maintenance and drainage rather than setting verification. Thus, filling the boiler to the normal operating level directly relates to ensuring the gauge glass functions as intended.