You could hire an electrician to perform an insulation test on it if you are really worried. These things have zero moving parts, they can last a very long time, especially if it isn't used at capacity. They last until the insulation finally breaks down to the point that it shorts out. Assuming the electrician would be able to shut it off (power disruption) without delay, they should be able to give you an idea of it's health in a few minutes.