refining .gitignores

How do I get a list of files (in HEAD, say) that would be ignored by the .gitignore files (and the other usual settings)?

It feels like something like this ought to work:

     git ls-files -z | xargs -0 git ls-files --ignored

But listing its arguments that are ignored by .gitignore (etc.) doesn't seem to be what "git ls-files --ignored" does. Or at least, not quite as straightforwardly as that.

The motivation is (obviously) that I fear some of the .gitignore patterns are too broad, and a reasonable check is that none of the files that are already committed would be caught by the patterns.