A standby set that has been started once a month and never asked to carry the building is a set nobody has tested. The two things that fail are almost never the engine.
The first is the changeover switch, which on about half the sets we see has never been operated since installation and does not move cleanly. The second is the battery, which dies quietly and is only discovered on the night of a real outage.
So we operate the changeover on and off under load, put the battery under a proper load test rather than reading its voltage, and write every figure on the job sheet with the date. The recording is what makes next year's visit useful. We do not sell generators, so there is nothing we gain by condemning yours.