“You don't have a voltmeter. Do you have a potato?”
Another neat trick in the same ballpark (but not involving a potato :-p) is to create a 220v to 110v adapter with a "resistance bridge".Plugging two (hopefully pretty equal resistance) incandescent lights in series, each lamp should have a 110v potential difference... say:A ----(60W lamp)-- B --(60W lamp)----- CBy plugging A and C "cables" to a 220V electrical outlet, one should be able to grab 110V from either A-B or B-C :-) (assuming both lamps have about the same resistance). Most electronic devices work with a wide range of input voltage any way, so it doesn't have to be extremely precise.