A few years ago I spent a couple of hours and a couple of rolls of 120 film testing my filters to determine my own factor. The procedure I used is as follows. I chose a landscape subject with a full range of tones and using my spotmeter metered and exposed for the shadows in my normal way with no filter in place. This negative is my reference. I metered the scene through the filter being tested and made a second exposure and then underexposed by one stop and over exposed by one stop. Obviously the last three exposures were made with the filter in place. I followed this procedure using 5 filters, red, orange, yellow,green and polariser. When the film was processed I selected the negative showing the shadow detail that matched the reference negative.
This method may not be the most scientific and does not take into account the effect that the filter has on the cells etc., but it works for me. If I replace a filter I carry out the same test on the new one.