the microwave oven is usually working in cw (continious wave) mode using an rotating dipol antenna at the top. Frequency is S-Band (~2.3GHz).
You could implement a small receiving antenna probe on the side wall to measure the energy. The antenna structure should be robust, isolated (ceramics maybe) before flashovers and heat resistant.
You should be aware that you measure field intensities in the near field with imaginary parts.