Start with the age: find the rating plate, decode the manufacture year (the first digits of the serial on most brands), and do Gulf Coast math - an attic tank past year 8 is on borrowed time, past 10 is on luck. Attic heat accelerates everything that kills tanks.
Check the pan and its drain line twice a year: the pan only helps if its drain actually runs somewhere and is not clogged with insulation or wasp nests. A water alarm in the pan (a $15 puck that screams when wet) is the cheapest ceiling insurance sold anywhere.
Replace on your schedule, in a mild month, with a permit - not on the tank's schedule in August with a wet ceiling. The price difference between those two events is mostly not the tank.
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