The extension of the standard model with new high-scale weakly coupled physics involving right-handed neutrinos in an effective field theory framework (SMNEFT) allows for a systematic study of heavy neutrinos phenomenology in current and future experiments. We exploit the outstanding angular resolution in future lepton colliders to study the sensitivity of forward-backward asymmetries to discover the possible single production of heavy Majorana neutrinos via e+e−→Nν, followed by a purely leptonic decay N→μ−μ+ν or a semi-leptonic decay N→μ−jj, for masses mN>50 GeV. In this regime, we consider the N production and decays to be dominated by scalar and vectorial four-fermion d=6 single NR operators. This is an alternative analysis to searches using displaced vertices and fat jets, in a higher mass regime, where the N is short-lived but can be found by the angular distribution of its decay products. We find that a forward-backward asymmetry between the final muons in the pure leptonic decay mode provides a sensitivity up to 12σ for mN=100 GeV, for effective couplings α=0.2 and new physics scale Λ=1 TeV. In the case of the semi-leptonic decay, we can compare the final muon and higher pT jet flight directions, again finding up to 12σ sensitivity to the effective signals.