Hold it up to your eyes like a mask. Open your eyes and look through a part of the bag that is silvery on both sides. Take it off your eyes and hold it over your hand. Try to see your hand through the plastic. The side of the plastic that has more light is going to win.
Bright light can travel through the plastic to the darker side.
If you are in the dark, you'll be able to see the light coming through. But if you're on the the brighter side, the weak light coming through is so dim, you won't be able to notice it. The bag will look shiny and like a mirror. Find a place with an outside light fixture near a window screen. Turn on the outside light and go outside.