  General relativistic magnetohydrodynamic (GRMHD) simulations of accretion
disks and jets associated with supermassive black holes show variability on a
wide range of timescales. On timescales comparable to or longer than the
gravitational timescale $t_G=GM/c^3$, variation may be dominated by orbital
dynamics of the inhomogeneous accretion flow. Turbulent evolution within the
accretion disk is expected on timescales comparable to the orbital period,
typically an order of magnitude larger than $t_G$. For Sgr A*, $t_G$ is much
shorter than the typical duration of a VLBI experiment, enabling us to study
this variability within a single observation. Closure phases, the sum of
interferometric visibility phases on a triangle of baselines, are particularly
useful for studying this variability. In addition to a changing source
structure, variations in observed closure phase can also be due to interstellar
scattering, thermal noise, and the changing geometry of projected baselines
over time due to Earth rotation. We present a metric that is able to
distinguish the latter two from intrinsic or scattering variability. This
metric is validated using synthetic observations of GRMHD simulations of Sgr
A*. When applied to existing multi-epoch EHT data of Sgr A*, this metric shows
that the data are most consistent with source models containing intrinsic
variability from source dynamics, interstellar scattering, or a combination of
those. The effects of black hole inclination, orientation, spin, and morphology
(disk or jet) on the expected closure phase variability are also discussed.
