Pop wrote:

There are even instruments that can tell the thickness of a
tank's walls without any cutting, drilling or damage. DeFelsco
is one instrument maker that comes to mind. Fairly expensive
though.
....

Generally, one can tell pretty well simply w/ a "tat-a-tat" w/ a small
hammer---the sound will change in any place w/ a significantly thinner
wall or heavy rust scale.

As noted above, the likely place is the bottom where condensation tends
to collect and sit. If there's an accessible drain, simply pulling it
for a small sample can tell a lot, too.