How about deliberately putting a strong magnet close to the rotating test masses, then repeating the experiment with the magnet on the other side and verifying that it doesn't make a difference?
Of course, that might be the easiest way. Given that you can get tiny magnets that are pretty strong but only a millimeters or so in size and therefore also light, you could just stick them to your different test masses in various configurations and see if it makes any difference while leaving the mass distribution pretty much unchanged.
