The Water Heater in the Report

InterNACHI covers the equipment, the energy source, venting connections, the temperature and pressure-relief valve and seismic bracing, and requires the labeled capacity to be described. ASHI covers the equipment, its energy sources and its vent systems and chimneys.

Source: InterNACHI Home Inspection Standards of Practice; ASHI Standard of Practice.

InterNACHI states the inspector is not required to measure the water heater’s capacity, temperature or adequacy, nor to test safety valves; ASHI likewise does not require relief valves to be tested.

So the relief valve is looked at. Nobody lifts the lever. A report that records the valve as present has told you it is present.

Because on an older unit a valve that has never been lifted may not reseat afterwards, and an inspector who lifts it can turn an observation into a leak in somebody else’s house. The standards decline the test for a reason.

The labeled capacity, because InterNACHI requires it to be described and it is the one number that tells you whether the unit suits the house. And the discharge pipe from the relief valve: where it terminates, and whether there is any sign it has ever discharged.

Neither standard asks anyone to estimate remaining life, and no source this site would cite publishes a defensible figure for it. The manufacturing date is usually decodable from the serial number, which is a fact; anything beyond that is an opinion.