tracking down a leak

Yes, I have some Co/AR welding gas in a small tank. I can just chuck it up and seep it out until I see 250 psi? Do I pressurize it when running or off? Thanks. The tubes that go back to the rear under the car look pretty chalky and im sure there could be some corrosion down there as the car came from NJ and most of the AL parts, even in the engine bay have what looks like road salt pock marks.