You can check to see if they block polarized light such as the light reflected off water. If you had 2 pairs of sunglasses, you could perform the experiment shown in Figure 24–44 to check if the glasses were polarizing.
Work Step by Step
Skylight is polarized, so you could take the sunglasses and view the sky while you rotate the sunglasses, and check if the sky lightens and darkens as you do so. You could also use other sources of polarized light, such as a liquid crystal display or reflections from a lake.