Something to try (using Acrobat 9 Pro).
Disclaimer - I'm in Windows; however this may work on your Mac.
Create a new Batch Sequence.
In the dialog "Edit Batch Sequence - <the name you assigned>" click the "Select Commands..." button.
Select and Add "Accessibility Checker".
Click the "Edit" button.
The "Accessibility Full Check" dialog is presented.
Be sure that "Create Accessibility Report" is ticked.
Use "Choose..." to set a folder for where the report is to go.
(n.b., report is an HTML file)
With a Folder identified click on the button "Start Checking".
No, Full Check won't "run" -- this makes the Folder choice "sticky".
Use Checking Option "Adobe PDF".
Close the dialog.
"OK" out of the "Edit Sequence" dialog.
"OK" out of the "Edit Batch Sequence - <the name you assigned>" dialog.
Run the Batch Sequence on two test PDFs (one known to be tagged, one known to be not tagged).
Upon completion Acrobat presents a "Warnings and Errors" dialog.
If a PDF is not tagged this is identified.
The report (HTML) file holds this information as well and is present in the Folder that you choose.
You can view the HTML report, cull from it, convert it to PDF for programmatic harvest, or whatever.
Keep in mind that "Tagged" does not mean a PDF is "good-to-go".
While there is "programmatic valuation" routines available they provide a rather high level perspective.
They do not (yet) provide the close look by a warm body that, currently, is required to assure that what is in the structure tree is actually correct. As well, some make unspoken premises as to what is "correct" that are somewhat "off".
Example: The grouping element "Document" is not the only allowed grouping element (discussed in ISO 32000-1) that can serve as a valid structure tree root element.
In short, a knowledgeble peson will have to assure your PDFs are well-formed Tagged PDF.
On a positive note ISO 14289-1 (PDF/UA) is in "FDIS" and it is anticipated that it'll be on-the-street in a few months.
Having a normative Standard will most useful.
It's easy when you know how - see this blog post for instructions and a profile.
I was hoping I could use a Preflight check for this -- however, there isn't one called "PDF has accessibility tagging", and I don't know what to look for.