  We consider an extension of the Standard Model with two right-handed singlet
fermions with mass at the electroweak scale that induce neutrino masses, plus a
generic new physics sector at a higher scale $\Lambda$. We focus on the
effective operators of lowest dimension $d=5$, which induce new production and
decay modes for the singlet fermions. We assess the sensitivity of future Higgs
Factories, such as FCC-ee, CLIC-380, ILC and CEPC, to the coefficients of these
operators for various center of mass energies. We show that future lepton
colliders can test the cut-off of the theory up to $\Lambda \simeq 500 -
1000\;$TeV, surpassing the reach of future indirect measurements of the Higgs
and $Z$ boson widths. We also comment on the possibility of determining the
underlying model flavor structure should a New Physics signal be observed, and
on the impact of higher dimensional $d=6$ operators on the experimental
signatures.
