If Ohmic heating is dominant, then your heating power is P = I*U (on the temporal average)

you can measure the weight of your electrode and if you know the heat capacity of your material, you can determine the temperature rise according to Q = c m dT

Please see Radio-Frequency Capacitive Discharges Raizer, Yuri P., Shneider, Mikhail N., Y ISBN 10: 0849386446 / ISBN 13: 9780849386442 Published by CRC Press, 1995 for theory and technology of capacitively coupled RF discharges. Another relevant book is Plasma Chemistry, by Alexander Fridman, Drexel University, Philadelphia, 2008.

Ohmic heating is a rough zero order estimation. Near electrode processes play a key role in electrode heating of CCP-RF discharges. Thermal flux to the electrodes may exhibit non linear dependence on pressure and power density.

The difference between grounded and powered electrodes may rise from the difference in current densities on the electrodes - some grounded parts of your chamber may also work as electrode decreasing current density on the grounded electrode.

there are many techniques can used to determine the electron such as Ac Langmuir probe and spectroscopy such as Boltzmann plot method or ratio method