Depended on what this machine is the quick way is drill a hole and then weld it shut after. if the item is solid the drill bit will feed normally. If there is a void the drill bit will break through, this will also draw out any material it is filled with.because you donu2019t want to cut it open drilling probably isnu2019t a option. If you have the money or equipment you can X-ray the item.Personally on items that I didnu2019t want to damage, I would weigh the components. If youu2019re sure that it is mild steel you can work out the volume of the item if it is similar in weight to the same volume of steel then it is solid.another option is to hit it and listen. Solid steel has a dire threat note compared to a empty space, or a space filled with a different material to steela picture of the item could also help if people new what it was they could give you an answer directly.