You could mark all posts read then go to Settings => "Filters & streams" then edit filter and uncheck feed to see what posts were filtered out (they will become new unread posts). Then you could edit filter and check feed back again.

Or you could edit filter and change its query from
some query
to
NOT (some query)
then filter will be inverted.

Perhaps I could add some debug view mode that only shows posts that are filtered out but can't say when I get to it.