Observe from IR/Raman spectra whether molecule is in a local minimum on PES

I have done a geometry optimization of *cis* and *trans*-difluoroethene using RHF/STO-3G in Gaussian03. Although this is a simple method and a small basis set, this should ideally put place the molecule in a minimum on its potential energy curve. Within this minimum are vibrational states. I am wondering if it is possible to see, from the calculated IR and Raman spectra whether the optimisation was successful, i.e. whether the molecule was indeed placed in a minimum.


I am thinking like this: If the molecule is in a minimum, then all vibrational IR and Raman active vibrational states should be visible (given a high enough activity, of course) in the theoretical spectra. However, if the molecule is not in the minimum, then only those vibrational states of *higher energy* should be visible. Hence, fewer IR/Raman active vibrational modes would be observed if the geometry optimization was not complete.


According to [this](https://chemistry.stackexchange.com/questions/14413/how-are-the-frequencies-at-a-local-maximum-of-pes-like) question, all frequencies at a local minimum would be positive, and all negative on a maximum. I assume then that somewhere in-between, you would have both negative and positive frequencies. However, given *not* the table of IR/Raman active modes and their frequencies, but the actual spectra instead, would I then be able to decide if the molecule is in a minimum or not?


