For active beam manipulation devices, such as those based on liquid crystals, phase-change materials, or electro-optic materials, measuring accumulated phase of the light passing through a layer of the material is imperative to understand the functionality of the overall device. In this work we discuss a way of measuring the phase accumulation through a switched layer of Ge_2Sb_2Te_5, which is seeing rapid use as means to high speed dynamic reconfiguration of free space light. Utilizing an interferometer in the switching setup and modulating the phase of one arm, the intensity of a probe beam can be captured and phase data pulled from it. Simulations were used to discover the connection between the intensity modulations and the phase information. The technique was tested experimentally and it was found that within error, the measurement was robust and repeatable.