The identification of the decay pathway of the nucleobase uracil after being photoexcited by ultraviolet light has been a long-standing problem. Various theoretical models have been proposed but yet to be verified. Here, we propose an experimental scheme to test the theoretical models of gas phase uracil decay mechanism by a combination of ultrafast x-ray spectroscopy, x-ray diffraction, and electron diffraction methods. Incorporating the signatures of multiple probing methods, we demonstrate an approach that can identify the dominant mechanism of the geometric and electronic relaxation of the photoexcited uracil molecule among several candidate models.