  We present synthetic continuum and $^{13}$CO and C$^{18}$O line emission
observations of dense and cold filaments. The filaments are dynamically evolved
using 3D-MHD simulations that include one of the largest on-the-fly chemical
networks used to date, which models the detailed evolution of H$_2$ and CO. We
investigate the reliability of observable properties, in particular filament
mass and width, under different simulation conditions like magnetic field
orientation and cosmic ray ionisation rate. We find that filament widths of
$\sim$0.1 pc can be probed with both line and continuum emission observations
with a high accuracy (deviations $\leq$ 20%). However, the width of more narrow
filaments can be significantly overestimated by up to a factor of a few. Masses
obtained via the dust emission are accurate within a few percent whereas the
masses inferred from molecular line emission observations deviate from the
actual mass by up to a factor of 10 and show large differences for different
$J$ transitions. The inaccurate estimate of filament masses and widths of
narrow filaments using molecular line observations can be attributed to (i) the
non-isothermal state of the filaments, (ii) optical depth effects, and (iii)
the subthermally excited state of CO, while inclination effects and opacity
correction only influence the obtained masses and widths by less than 50%.
Both, mass and width estimates, can be improved by using two isotopes to
correct for the optical depth. Since gas and dust temperature generally differ
(by up to 25 K), the filaments appear more gravitationally unstable if the (too
low) dust temperature is used for the stability analysis.
