Relativistic shocks have been widely studied as promising sites for ultra-high-energy particle acceleration. Predictive results have been obtained from both analytical and numerical methods but require tests from observations. The gamma-ray burst (GRB) multi-wavelength 'afterglow' emission is believed to be produced in relativistic shocks where high-energy electrons are accelerated. Afterglow observations thus provide information to test our current understanding of shock acceleration. Here we focus on six GRBs with determined redshifts, selected for having XRT afterglow detection at $\sim$ $10^7$ seconds after the GRB trigger time. We show that these observations lead to meaningful constraints on the maximum electron energy in the underlying shocks.