Single photon sources are an important component in many quantum information processing systems. Despite their importance, current modeling methods predominantly focus on developing optimistic performance bounds, and are not conducive to performing engineering optimization studies. To enhance the capability of modeling methods, we propose a process to utilize the classical electromagnetic dyadic Green's function of a system as an integral component of determining the emitted photon states produced by a single photon source. Importantly, the Green's function can be readily computed using standard fullwave numerical methods, making it a viable analysis tool. We demonstrate this process by characterizing the emission of a microwave frequency single photon source that uses a transmon as a quantum emitter, and find qualitative agreement with a fabricated device that has similar design characteristics.