Most fuel tanks sit where nobody looks: under the cockpit sole, behind the engine, sometimes foamed into the hull. An aluminium tank lasts well dry. Hold water against it, in the bilge, under a strap or in foam that's soaked up moisture, and it pits from the outside until it leaks.
How a surveyor checks it
See as much of the tank as you can. The top and the fittings first: corrosion, weeping round the sender and fill, and the fittings themselves. The surveyor in Good Old Boat's account warned never to use brass fittings with aluminium tanks; use stainless.
Then the bottom and supports, where the water collects. That's where Good Old Boat's tank had pitted. Its surveyor condemned it on sight: an aluminium tank sitting in salt bilge water for 23 years means corrosion. Smell for fuel and look in the bilge below.
Foam matters. US rules set limits on how much water foam round a metal tank may absorb, in 33 CFR 183.516, because foam that holds water holds it against the tank.
What decides the grade
Fuel outside the tank is an A. Deep pitting, a tank sitting in water or brass fittings on aluminium is a B. A tank that's mostly hidden, with what's visible in fair order, is a C, and the finding says what couldn't be seen.