There are a couple of ways you can test this:
One way is to hold one lens in front of the other so that they are parallel. While looking through the back of both of them, rotate one so that the lenses cross like a “T.” At a certain angle they will black out.
You can also hold the lenses in front of a computer screen. Tilt the lenses back and forth at 90° angles, and if polarized, they'll darken as you rotate them.