I have found this:

5% Cupper sulfate CuSO4 is a good solution for that, where it forms black
color with zinc and red with steel.

So i will test with some on a cotton swab and check which color it turns,
black is zinc.

"Dave Hinz" wrote in message
...
On Thu, 27 Apr 2006 10:35:40 -0400, Andre Langevin
wrote:
Hi all,

I have a old 1967 fuel tank on my boat that was used for gasoline and i'm
converting to diesel. I'm wondering though if the tank "could be" coated
with zinc inside (galvanized). First i wonder how someone could put
zinc
inside a tank but anyway do you have an idea on how this could be checked
?
The tank is closed except for one small 2 inches wide hole.

Um, look inside to see if it looks like zinc maybe? Or pay someone who
knows how to handle gasoline tanks to coat it with something compatible
with diesel? Gasoline tanks are nothing to fool with if you're
untrained.