  Class 0 objects, which are thought to be the youngest protostars, are
identified in terms of NIR or radio emission and/or the presence of molecular
outflows. We present combined hydrodynamic and radiative transfer simulations
of the collapse of a star-forming molecular core, which suggest two criteria
for identifying dense cores with deeply embedded very young protostars that may
not be observable in the NIR or radio with current telescopes. We find that
cores with protostars are relatively warm (T>15K) with their SEDs peaking at
wavelengths <170 micron, and they tend to appear circular.
