A list of PDF/A validators is on the pdfa.org web site here:
verapdf
A free online PDF/A validator is available here:
http://www.validatepdfa.com/
A report on the accuracy of many of these PDF/A validators is available from PDFLib:
https://www.pdflib.com/fileadmin/pdflib/pdf/pdfa/2009-05-04-Bavaria-report-on-PDFA-validation-accuracy.pdf
Se as well:
https://www.pdflib.com/knowledge-base/pdfa/