We present a new approach for inferring the electric field distribution inside materials in multimode cavities with mode stirrers. We calculate the electric field in the dielectric material by a two-dimensional modeling of a typical multimode microwave applicator with some mobile metallic sheets. We compare simulated results with classical approaches, such as Lambert's law or a constant electric field distribution. The proposed method allows for a better understanding of how these structures can be applied for heating materials when computing the microwave energy absorption in the dielectric. Finally, we perform experimental tests in a microwave multimode oven for validating purposes.