Nitrogen is produced by CNO-cycling in massive stars, and can be ejected in significant amounts in supernova explosions. While in H-rich SNe, its [N ii] 6548, 6583 emission becomes obscured by strong Hα⁠, in explosions of He stars, this nitrogen emission becomes more visible. We here explore the formation of this line, using the sumo code to compute spectra for a grid of 1D models with parametrized mixing informed from new 2D simulations. Because the mass fraction of nitrogen in the ejecta decreases with larger He-core masses, as more of the He/N zone gets processed by shell helium burning and is lost to winds, the [N ii] luminosity relative to the overall optical flux probes the He-core mass. By comparing to large samples of data, we find that low-mass He cores (⁠M_preSN≲ 3 M_⊙⁠) are exclusively associated with Type IIb SNe, with the exception of Type Ib SN 2007Y. Seeing no strong nitrogen emission in other Type Ib SNe, the implication is either an origin from low-mass stars with the He/N layer (but not the He/C) layer peeled away, or from higher mass He cores. We also see no clear nitrogen emission in Type Ic SNe. We discuss the diagnostic potential of this new line metric, and also dependencies on mass-loss rate and metallicity.