  Direct observational measurements of the magnetic field strength in
prestellar cores typically find supercritical mass-to-flux ratios, suggesting
that the magnetic field is insufficient to prevent gravitational collapse.
These measurements suffer from significant uncertainties; an alternative
approach is to utilise the sensitivity of prestellar chemistry to the
evolutionary history, and indirectly constrain the degree of magnetic support.
We combine non-ideal magnetohydrodynamic simulations of prestellar cores with
time-dependent chemistry and radiative transfer modelling, producing synthetic
observations of the model cores in several commonly-observed molecular lines.
We find that molecules strongly affected by freeze-out, such as CS and HCN,
typically have much lower line intensities in magnetically subcritical models
compared to supercritical ones, due to the longer collapse timescales.
Subcritical models also produce much narrower lines for all species
investigated. Accounting for a range of core properties, ages and viewing
angles, we find that supercritical models are unable to reproduce the
distribution of CS and N$_2$H$^+$ line strengths and widths seen in an
observational sample, whereas subcritical models are in good agreement with the
available data. This suggests that despite presently having supercritical
mass-to-flux ratios, prestellar cores form as magnetically subcritical objects.
