  The relationship between galaxy characteristics and the reionization of the
universe remains elusive, mainly due to the observational difficulty in
accessing the Lyman continuum (LyC) at these redshifts. It is thus important to
identify low-redshift LyC-leaking galaxies that can be used as laboratories to
investigate the physical processes that allow LyC photons to escape. The
weakness of the [S II] nebular emission lines relative to typical star-forming
galaxies has been proposed as a LyC predictor. In this paper, we show that the
[S II]-deficiency is an effective method to select LyC-leaking candidates using
data from the Low-redshift LyC Survey, which has detected flux below the Lyman
edge in 35 out of 66 star-forming galaxies with the Cosmic Origins Spectrograph
onboard the Hubble Space Telescope. We show that LyC leakers tend to be more [S
II]-deficient and that the fraction of their detections increases as [S
II]-deficiency becomes more prominent. Correlational studies suggest that [S
II]-deficiency complements other LyC diagnostics (such as strong Lyman-$\alpha$
emission and high [O III]/[O II]). Our results verify an additional technique
by which reionization-era galaxies could be studied.
