Question

Two closed containers look the same but when is packed with LED and the other with a few feathers how could you determine which has more mass if you end the containers were orbiting in a weightless condition in outer space

Answer 1

You would want to push both containers, first one and then the other,
with exactly the same strength of push. 

The one that takes off slower (with less acceleration from the same
net force) is the one with more mass.

Answer 2

The lead has more mass because it is denser however the weights are equal in outer space