line up two lenses and look through them. Rotate one. If it doesn't get darker & lighter, then at least one lens isn't a polarized lens. If you don't have two pairs of lenses then rotating one pair of polarized glasses whilst looking through them at an LCD screen will work fine, eg your computer screen or even your digital watch face. Rotate the lens clockwise through 90-180 degrees and the screen/watchface will go dark and then light up again as you keep rotating.
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