Two closed containers look the same, but one is packed with lead and the other with a few feathers. How could you determine which has more mass if you and the containers were orbiting in a weightless condition in outer space?

You could push the containers, applying roughly the same amount of force. The container that accelerates more, going at a greater speed under the action of the applied force will the the container that has the feathers as F = ma, and the container with the feathers will have a significantly less mass than the container with the lead.