How do I show the changes which have been staged?

It should just be:

git diff --cached

--cached means show the changes in the cache/index (i.e. staged changes) against the current HEAD. --staged is a synonym for --cached.

--staged and --cached does not point to HEAD, just difference with respect to HEAD. If you cherry pick what to commit using git add --patch (or git add -p), --staged will return what is staged.

A simple graphic makes this clearer:

git diff

Shows the changes between the working directory and the index. This shows what has been changed, but is not staged for a commit.

git diff --cached

Shows the changes between the index and the HEAD (which is the last commit on this branch). This shows what has been added to the index and staged for a commit.

git diff HEAD

Shows all the changes between the working directory and HEAD (which includes changes in the index). This shows all the changes since the last commit, whether or not they have been staged for commit or not.

Also:

There is a bit more detail on 365Git.

Note that git status -v also shows the staged changes!(meaning you need to have staged -- git add -- some changes. No staged changes, no diff with git status -v.
It does that since Git 1.2.0, February 2006)

In its long form (default), git status has an undocumented "verbose" option which actually display the diff between HEAD and index.

And it is about to become even more complete: see "Show both staged & working tree in git diff?" (git 2.3.4+, Q2 2015):

git status -v -v