How to find joists through sheetmetal ceiling?
You might be able to pull the carpet a bit in a corner of the room.  Perhaps you can find nails holding down the subfloor to the joists.  A good place to start would be a closet where it would be easy to hide and tack back down.  From this point it might be easier to guess what the typical joist spacing would be.

Typically ceiling joists will be spaced 12", 16", 19.2" or 24" OC.  Rather than damage the ceiling it would be easier to pull up some carpet upstairs and try driving a nail or screw at each of these lengths and see which length grabs a joist.