The issue here is related to what is actually hidden and what is not hidden but not visible due to the parent post being hidden. The latter, really complicates stuff so I'll go for the former.
In order to get content that has been explicitly hidden by a user, you can run this SQL query:
SELECT COUNT(*) FROM `qa_posts`
WHERE `type` IN ('Q_HIDDEN', 'A_HIDDEN', 'C_HIDDEN');
If you want to get the results of Questions, Answers or Comments only, just remove them from the query.