Ghostscript isn't a validation tool, it won't tell you if a PDF file is a PDF/a file. Not least because Ghostscript doesn't care.
[edited in answer to the comment below]
You have not specified a ColorConversionStrategy, specifying ProcessColorModel is not sufficient (and indeed not required). For PDF/A you (generally) will need to supply a ColorConversionStrategy unless you know for absolute certain your input file exclusively uses device independent colour spaces.
Your command line dos not appear to include a pdfa_def.ps input file. This file is important (and you must modify it appropriately), if you don't supply this, or equivalent programming, then your PDF/A file will not include an OutputIntent and, if it uses any device colour space, will not be valid.
Don't (really, seriously, don't) use -dNOSAFER. For PDF input there shouldn't be any need to do this and it's opening you up to security vulnerabilities.
Your command line includes -c -quit
which is incorrect and should cause an error. You really mean -c quit
and you don't need to do that anyway. Just leave it out.
The documentation on creating a PDF/A file can be found online here
or in ghostpdl/doc/VectorDevices.htm#PDFA Note the 4th bullet point;
Specify a PDF/A definition file before running the input document.