The late-time optical/radio afterglows of γ-ray bursts (GRBs) are believed to be synchrotron emission of electrons accelerated in relativistic collisionless shocks propagating in the ambient medium of the sources. However, the fraction f of electrons that are coupled to protons and accelerated remains unclear and a large number of thermal electrons that are not coupled to protons may be left behind. If f<1, the true explosion energies of GRBs are f -1 times larger than those commonly estimated with f=1. Thus the value of f gives an important constraint on the nature of the central engine of GRBs and the physics of collisionless shocks. Although early-time radio observations can probe the thermal electrons, they are difficult at present. We show that the Faraday rotation effects of the thermal electrons may suppress the linear polarization of the afterglow at frequencies higher than the absorption frequency in the late time, if the magnetic field is ordered at least in parts, and that f can be constrained through the observation of the effects. We find that those effects may be detected with late-time, ≥1 day, polarimetry with ALMA for a burst occurring within 1 Gpc (i.e., z≃0.2), if f∼10 -1.