How to find out in a post-commit-hook if the current revision is creating a branch/tag?

Question

I want to write a post-commit hook that only triggers, if the current commit is creating a new tag. Tags are copied to a /tags/*new_tag_name*/ directory. My motivation is, if the commit is creating a new tag, the new tag should be checked out in my web root directory.

How can I do this in the post-commit script, where I only have the svn name and revision number as function parameters?