[asphaltgambler]

Depends on if the unit was from a big box retailer or a commercial model. Usually the heater elements fail first, the lower one from sediment adhesion. the tanks will rust from the inside / bottom and will develop a 'drip' to let you know. They make water level alarms where the tank is inside a shallow runoff bowl and a the alarm is battery powered with a level sensor at the bottom.

At that age, if nothing is wrong, I would install a level alarm and forget it until something happens. Prices have gone waaay up and availability for some units are weeks out.