  Determining the importance of magnetic fields in star forming environments is
hampered by the difficulty of accurately measuring both field strength and gas
properties in molecular clouds. We post-process three-dimensional non-ideal
magnetohydrodynamic simulations of prestellar cores with a time-dependent
chemical network, and use radiative transfer modelling to calculate
self-consistent molecular line profiles. Varying the initial mass-to-flux ratio
from sub- to super-critical results in significant changes to both the
intensity and shape of several observationally important molecular lines. We
identify the peak intensity ratio of N$_2$H$^+$ to CS lines, and the CS $J=2-1$
blue-to-red peak intensity ratio, as promising diagnostics of the initial
mass-to-flux ratio, with N$_2$H$^+$/CS values of $>0.6$ ($<0.2$) and CS
blue/red values of $<3$ ($>5$) indicating subcritical (supercritial) collapse.
These criteria suggest that, despite presently being magnetically
supercritical, L1498 formed from subcritical initial conditions.
