I'm a Valkyrie in Unnethack, and pretty early on (dungeon level 3) I've run across a large box containing a gray stone. It's the first gray stone I've seen, so I don't want to pick it up in case it's a loadstone.
The only way I can think to get the stone out so I can kick-test it is to lock the box and then try to #force it open with a blunt weapon, hoping that I destroy the box in the process. Trying that is a little ways off as I haven't found any tools yet (or a Wand of Wizard Lock), so at this point I can't lock it. Even then, it's a gamble whether or not the box will be destroyed in the #force attempt.
Are there other options? I'm hoping maybe I could apply a pickaxe or mattock to the square the box is in to destroy the box, but I doubt it will work. I'll test when I find a pickaxe. I could also imagine zapping a Wand of Striking down while standing on the box doing the job–it breaks doors!–but I'm similarly doubtful. I've always thought that pushing a boulder over a box or bag should have a pretty good chance of destroying contents, but I know that's actually a nice safety precaution to keep your stash from being eaten/absorbed.
Best Answer
The loadstone curse doesn't work inside a box. (This is why the game won't let you put a loadstone in a container you're holding; otherwise, you could just bag it and drop it.) Thus, you can figure out whether there's a loadstone inside by picking up the whole box and seeing how heavy it is.
A large box with a loadstone in it has a weight of 850. A large box with any other kind of gray stone in it has a weight of 360. Figure out your carrying capacity, pick up or drop items until the difference in box weights will be noticeable as a difference in encumbrance status, and pick up the box.