  We uniformly analyze 136 optically detected PNe and candidates from the
GLIMPSE-I survey in order to to develop robust, multi-wavelength,
classification criteria to augment existing diagnostics and provide pure PN
samples. PNe represent powerful astrophysical probes. They are important
dynamical tracers, key sources of ISM chemical enrichment, windows into late
stellar evolution, and potent cosmological yardsticks. But their utility
depends on separating them unequivocally from the many nebular mimics which can
strongly resemble bona fide PNe in traditional optical images and spectra. We
merge new PNe from the carefully evaluated, homogeneous MASH-I and MASH-II
surveys, which offer a wider evolutionary range of PNe than hitherto available,
with previously known PNe classified by SIMBAD. Mid-infrared (MIR) measurements
vitally complement optical data because they reveal other physical processes
and morphologies via fine-structure lines, molecular bands and dust. MIR
colour-colour planes, optical emission line ratios and radio fluxes show the
unambiguous classification of PNe to be complex, requiring all available
evidence. Statistical trends provide predictive value and we offer quantitative
MIR criteria to determine whether an emission nebula is most likely to be a PN
or one of the frequent contaminants such as compact HII regions or symbiotic
systems. Prerequisites have been optical images and spectra but MIR morphology,
colours, environment and a candidate's MIR/radio flux ratio provide a more
rigorous classification. Our ultimate goal is to recognize PNe using only MIR
and radio characteristics, enabling us to trawl for PNe effectively even in
heavily obscured regions of the Galaxy.
