i think the right way would be to pressurize the tank to the stated pressure on the mfg tag (usually 3 psi) and see if it holds pressure. of course a visual external and internal is highly recommended, you don't know when the tank will let go. enclosed are photos of my original factory 60 gallon tank that still had 30 gallons of fuel when i purchased it. it was wrapped around with water soak foam

additional fuel tank photos

Thanks Jet. That's good info!