Gas tanks tend to rust through the seams, and around the welds, and they leak there if anywhere...but you will have a visible gas leak, and notice the smell of gasoline.
If the tank is rusting inside, the tank was left open at some point, and rust started above the gas line, where it may still be.
Remove the tank and fuel pump.
Duct tape the holes closed.
Put a hole in the tape for a compressed air nozzle to fit tightly in.
Spray compressed air into the tank, and spray soapy water on all the gas tank seams, if you have a leak, it will be visible.
I would inspect the inside of the tank with a small mirror and a light.
If there is a small amount of rust, get a gallon of Evaporust(biodegradable and non toxic), and let it soak in the tank for several hours.
A simple electrolosiss tank can be made to remove the rust too(google/youtube).
There is also seam sealer gas tank repair, kind of like JBWeld, which also works.
But, if you do have a rust hole, and do not clean or encapsulate All of the rust...then the fix will be short lived.
The entire inner tank can have a coating or sealer applied as well.
Worst case, the tank needs cut, fabricated, welded, bodyworked, and repainted, at which point unless those are skills that you have, it is Much cheaper and easier to buy a used gas tank.
