How can I determine the polarization direction of a polarizing film?

The polarizing direction, also known as the polarization axis, runs parallel to the first mentioned dimension of the polarizing film. For example, a film with the dimension 500mm x 1000mm would have the polarizing direction running parallel to the 500mm side. Square polarizers have the polarizing direction indicated by a sticker on the protective film. In both cases the tolerance is +/- 2°.