Take two pair of the sun glasses put one pair on and look at a bright light. Put the other pair in front of the first pair and rotate the lens to perpendicular to the first pair of glasses. It the polarization is good quality you will see almost no light through the crossed lenses. You will se some light particularly around the edges because the lenses are not flat and parallel to one another. This method is a good test of the quality of the polaroid in the lenses. I used to teach optics as one of my core courses when I was a professor