The current interest in research and development of solid electrolytes for battery systems dictates a necessity to evaluate their electrochemical stability in a wide potential range. It is supposed that the stability and properties of the interface formed between the electrode and solid electrolyte at the applied potential (the analog of solid electrolyte interphase (SEI) in liquid electrolytes) are of great importance for the battery operation. While the electrochemical techniques can provide the knowledge of a stability window of the solid electrolyte, a direct method, which helps to trace chemical changes, is still missing, due to the difficulty to reach the interface between the solid electrolyte and thick electrode material. In this paper, we propose to use two-layer graphene transferred directly on the solid electrolyte as the electrode transparent for photoelectrons. Such an electrode is thin enough to probe the interface by X-ray photoelectron spectroscopy to trace the occurring chemical changes. To demonstrate this possibility, we have investigated the electrochemical reduction of Li1.5Al0.5Ge1.5(PO4)3 (LAGP) glass-ceramic electrolyte by in situ XPS.