Unfortunately my answer to your question is very late, but perhaps it will help others in future.
Impedance spectroscopy (at different conditions, like temperature, humidity, etc.) will be the method of choice. But it is a relatively special method, which will need a sophisticated analysis.
You can get an introduction to this complex topic by the literature written by J.R. Macdonald like:
Y. Barsukov, J. R. Macdonald, in Characterization of Materials, vol. 2, (ed. E. N. Kaufmann),John Wiley & Sons, Inc., Chap. Electrochemical Impedance Spectroscopy, 2. edition.,2012, pp. 898–912.
Impedance Spectroscopy Theory, Experiment, and Applications (see link)
or others like:
Impedance Spectroscopy - Applications to Electrochemical and Dielectric Phenomena (by V. F. Lvovich)
or for your special topic:
Treatment of the Impedance of Mixed Conductors Equivalent Circuit Model and Explicit Approximate Solutions (see link)
Article Treatment of the Impedance of Mixed Conductors Equivalent Ci...
Book Impedance Spectroscopy: Theory, Experiment, and Applications