When a gamma-ray burst (GRB) occurs inside thick merger ejecta or deep within a star, the newly created jets must drill their way through the confining media before breaking out and emitting their energy. The interaction with the medium leads to the formation of a non-relativistic cocoon around the jet and an interface separating the two components. Once the jet breaks out, the interface material accelerates and forms a mildly relativistic sheath around the jet, which can radiate over a wide range of angles. Recent studies have shown that the interface can hold a substantial amount of energy, which can alter the observed light during the prompt and afterglow phase, thus reveling information on both the jet and medium properties prior to the jet breakout. In this work I will present a method to probe the jet structure and its interface by modeling the afterglow light, focusing on its linear polarization. The temporal evolution of the polarization signature is highly sensitive to the emission region geometry, and when modelled carefully, can give robust observational features of jets with varying structures.