The Cosmological Special Theory of Relativity (CSTR) relates a local, unexpanded spatial coordinate to the physically expanded coordinate of a Robertson-Walker universe through a cosmological expansion function τ (t0), and re-derives relativistic wave equations, electrodynamics, and general relativity within the resulting cosmological inertial frame (Yi, 2020, 2025). Companion reviews of this program have each found, for the specific quantities they consider — Yukawa interaction ranges and cross sections, Coulomb and hydrogen-like binding energies,Schwarzschild and Kerr-Newman horizon radii, parametrized post-Newtonian coefficients, and the fixed-epoch Lorentz transformation itself— that every physically measured quantity, once expressed in the physically expanded coordinate, recovers its ordinary, epoch-independentvalue, with the cosmological expansion function appearing only in the corresponding local-coordinate expression. This paper draws togetherthat recurring finding into an explicit observational assessment: because CSTR predicts no departure from ordinary physics for any process confined to a single cosmological epoch, the theory is not constrained, but also not yet tested, by existing single-epoch observations, including present bounds on the time-variation of the fine-structure constant and the proton-to-electron mass ratio from quasar absorption spectroscopy. We argue that a genuine test of CSTR requires a process that spans a cosmologically significant interval of time within a single observation — such as the redshift-drift (Sandage-Loeb) test, multi-epoch photon propagation, or cosmological particle creation — precisely the regime in which a companion review has shown that CSTR’s transformation law departs from a simple group structure. We summarize current observational bounds relevant to each sector of the theory and identify the redshift-drift test as the most concrete near-term observational program capable of constraining CSTR’s genuinely multi-epoch predictions.