Extending a molecular field model for the orientational order in the nematic phase, we calculate the ²H-NMR splittings for the achiral solute 8CB-d₂ in the twist-bend nematic phase formed by the achiral liquid crystal dimer CB7CB. We give an explanation for the enantiotopic discrimination observed in the spectra and comparison with experimental data allows us to provide quantitative estimates of the order parameters (pitch and conical angle) that characterize the director modulation of the twist-bend nematic phase.