Hello,
There's no tool or class to detect what tags Google Docs is not able to read properly. We recommend you to try the content one by one to get the exact element is returning the error (if you send the DOCX to contact[at]phpdocx.com we'll check it too using Google Docs).
MS Word has default styles and values that some tags may need to open DOCX files properly with Google Docs and other programs such as OpenOffice or LibreOffice. For example table widths; you need to specify a width for each table and column to show tables with their correct sizes when you open the document with LibreOffice or Google Docs (Ms Word calculates table widths automatically if their values are empty).
Regards.