Corrosion may give away a crack, and cracks may form as a result of corrosion also. Over-tightening a valve may ruin threads and add to the inherent tension of the dome. Old European cylinders with a conical valve stem may develop cracks due to transverse neck strain. Rolling a tank is a crude way to look for unbalance. The implications are uneven thickness or folds in the metal.
Eddy current is used extensively in checking the integrity of airplanes.
Since there's a lot of aluminum requiring structural strength, cracks are the thing to look for. Commercially made devices, the Visual Plus , the Simple Eddy , and the Visual Eddy Machines are available for checking scuba tanks. Since aluminum tank manufacturers are involved in the marketing of these devices, it's in their best interest to make the price right. There's one thing we can't check for, namely cracks that haven't developed yet. Another thing, don't forget the external surface.