Visually, an epoxy grout will be glossy whereas the cement-based grout should be flat. Also, mild acid test could be conducted using muriatic acid that can purchased from ACE or a similar hardware store. Follow all recommendations by the supplier of the acid. An acid will cause the grout, if cementitious, to be degraded and cause bubbles to appear from the deleterious chemical reaction. A polymer modified cement-based grout may not bubble or degrade rapidly due to the polymer in the grout, but that does not mean that it is an epoxy grout.