How would the photographer use the polarizing filter to find out the direction of polarization of the light coming from the blue sky? her only reference is the polarization axis of the filter.

The answer is she will have to rotate the filter until the light's intensity is maximum. The light's polarization is along filter's axis.

Explanation:

This process is a phsycal phenomenon called the process of scattering of light by a molecule. We can also call it  Rayleigh scattering.

Rayleigh scattering  is used to explain why during daytime the sky looks so blue, the sunset looks so red, and the clouds so white. Polarization can also be explained by rayleigh scattering.

If a photographer wants to take a picture of the blue sky, she uses a polarizing filter  to increase the ratio of the clouds' intensity of the blue sky.

To find the right direction, she will have to rotate the filter until the light's intensity is maximum. The light's polarization is along filter's axis.