  We present new diagnostic tools for distinguishing supernova remnants (SNRs)
from HII regions. Up to now, sources with flux ratio [S II]/H$\rm{\alpha}$
higher than 0.4 have been considered as SNRs. Here, we present the combinations
of three or two line ratios as more effective tools for the separation of these
two kinds of nebulae, depicting them as 3D surfaces or 2D lines. The
diagnostics are based on photoionization and shock excitation models (MAPPINGS
III) analysed with Support Vector Machine (SVM) models for classification. The
line-ratio combination that gives the most efficient diagnostic is: [O
I]/H$\rm{\alpha}$ - [O II]/H$\rm{\beta}$ - [O III]/H$\rm{\beta}$. This method
gives $98.95\% $ completeness in the SNR selection and $1.20\%$ contamination.
We also define the [O I]/H$\rm{\alpha}$ SNR selection criterion and we measure
its efficiency in comparison to other selection criteria.
