In Acrobat, open the Tools pane and select Quick Check in the Accessibility panel. This quick check will tell you if tags exist in the file. When the check is complete, a dialog box opens and tells you the accessibility status of your document.
In Acrobat Pro, you can also do a full accessibility check. Click Full Check in the Accessibility panel. Check Create Accessibility Report and Create comments in document. Scroll down the Full Check dialog box and under Checking Options, select any items you want to check. Click Start Checking. This generates a report and adds comments in the document. Any errors are reported in the comment notes.