There are a few ways of measuring the spectral response, but the most accurate is to use a spectrograph. If you're technically inclined you can build and calibrate one. An excellent resource for building your own can be found at
http://www.astrosurf.org/buil/ and
http://www.astrosurf.org/buil/lisa/lisa.htm.
Once you've completed the calibration process you can download
Visual Spec - free spectral analysis software. This will produce the spectral response graphs through each filter.
Once you're done, you can complete a comprehensive filter spectral response library such as this -
http://astrosurf.com/~buil/filters/curves.htm
Off topic; but for those DSLR users that are using filters - you may find some information presented here interesting -
http://astrosurf.com/~buil/us/test/test.htm