Preforma is a project co-funded by the European Union that makes available veraPDF, software which can confirm whether or not PDF file is in an archivable PDF/A format.
The software can be downloaded here:
Extract the files from the zip file which downloads and click on the 'verapdf-install.bat' file. This will run an installation wizard for the veraPDF software.
In the folder the software installs in, find the file named, 'verapdf-gui.bat' and double-click on it. A Java based program will run. Click 'Choose PDF' to select the PDF you want to check. Then select the PDF archive format you want the software to check for. Click 'Execute' when you're ready.
The software can generate a validation report which you show the aspects of the PDF which do not comply with the requirements for the PDF/A format.