If you do not want to open every workbook using Tableau Desktop, then you can look at extracting the "twb" file from each packaged workbook "twbx" using any unzip tool, and look for SQL keywords in it. Because of several special characters included in the text, it may be difficult to read the complete query without replacing/getting rid of those special characters. But to get the table/view being used, simply search for the SQL keyword "FROM", if need be using different case formatting, and you may be able to identify them. In the image below, the "FROM" keyword is highlighted in white and you can see the table names being used beside it. You can also search for other keywords like JOIN beside which a table name may be present.
Hope this helps.