I would like to know how to tell if a water-heater is approaching its
useful life and needs to be replaced soon. I don't want to wait for
its water tank to crack open and flood the basement before I take
action.

I was planning to examine the anode rod in my 4-years old water-heater
with the hope of replacing it periodically in order to extend its
service life. But my basement doesn't have enough head room to extract
the anode rod. Therefore, I am thinking of just waiting for it to die
before I replace the water-heater and all its connecting fittings to
flexible fittings (then I can disconnect them easily for periodic
checking the anode rod). How can I tell when I should replace the
water-heater?

Thanks in advance for any suggestion.

Jay Chan