You could drain the pressure and remove the discharge to the tank line and the fitting into the tank. Use a borescope to visually inspect the bottom of the tank. The rusted part will usually be on the bottom and fairly easy to see this way. Most pressure vessels need to be inspected at 10 year intervals like propane tanks. You could then if suspicious, remove the compressor and fill the tank with water. Use a long hose and a hand pump to pressure it up to 200 PSI. Stand behind something in case it does rupture. If it passes that you should be good for a while yet. Just make sure to use water to fill the tank and in the pump. Use a good pressure gauge on the pump so you know what pressure you are at.