Abstract Determining whether Callisto hosts a subsurface ocean, and the implications for material and energy transport through its hydrosphere, is of high interest to NASA’s Europa Clipper and ESA’s Juice missions. A recent reanalysis of Galileo magnetometer data strengthens the ocean hypothesis, but definitive confirmation awaits new measurements. We assess how Callisto’s interior properties and presence or absence of an ocean affect observable quantities, including tidal response and dissipation, rotation‐rate variations, and spin‐axis obliquity. In addition to the well‐known influence of an ocean, we find that these quantities are strongly affected by viscoelastic deformations. As a result, measurements of a single quantity will likely be insufficient to confirm the presence of an ocean, but the combination of two of them will provide a conclusive test. Based on the expected performances of Juice and Europa Clipper investigations, pairing Callisto’s tidal response with either tidal dissipation or obliquity will provide a robust test.