There are a lot of PDFs out there which claim to be PDF/A, but fail a real smoke test. That claim is just a tag in the file's metadata. That tag can make f.e. Acrobat Reader display a special hint when rendering it.
A check for real compliancy requires some rather expansive commercial "preflight" software. Currently I'm not aware of any Free utility to do that job. See also here for some test results: Isartor testsuite
You can use Ghostscript to (try to) convert PDF to PDF/A. How to do this is documented here.
But note: this document was updated only very recently [*]. Previous versions of Ghostscript's Ps2pdf.htm did mis-lead users to run a command that created PDFs claiming to be PDF/A but which failed real smoke tests.
How to convert PDF to PDF/A with Ghostscript:
Here is a commandline:
-o output_pdfa.pdf ^
[*] Note: The problem lays with the parameter
PDFA_def.ps. This is a file you need to edit to suite your needs. Ghostscript ships with a sample of it in its
/lib subdirectory. This sample will not work as-is without you editing it. How to edit is inside the sample's comments.