Ganymede is the largest moon in our solar system, and unique in producing its own magnetic field, as well as possibly possessing a subsurface ocean. The data analysis from the Galileo spacecraft provided two models for the internal field—a dipole and quadrupole model or a dipole and induction model. The latter model is preferred due to the number of parameters being less than the former, given that the model discrepancy with respect to the measurements is similar for both. While the recently launched JUpiter ICy moons Explorer (JUICE) mission will focus on Ganymede in its orbital phase providing an in-depth analysis, there are flybys initially around the moon that can be used for internal field studies. We focus on the first three close flybys of the mission in order to assess how effective the expected observation would be for distinguishing between the induction and quadrupole signals and confirm the existence of an ocean. We begin with an analysis of the JUICE and Galileo trajectories in different reference frames and predict the induction signal using the time varying field of Jupiter. A comparison between the two signatures for Galileo flybys agrees with previous results indicating induction to be present. Finally, we display and discuss the feasibility and importance of the three JUICE flybys to observe the induction field and hence confirm the ocean.