Are you worried about damaging it? Or thinking it will explode if hit?
Many a welder has gotton a surprise when he welded a "empty tank" Whether it be propane , gasoline etc the gas is usually mixed with some air and becomes very explosive !
Usually leaves a residue as well. I had to punch holes in a few old tanks to get a scrapyard to take them - they had been empty for a very long time, but still had a very strong odor