Hook into single post display?

Anybody know how to determine when a single post is being displayed, as opposed to the home page where multiple pages are displayed?

I want to add a filter for special processing when viewing a single post. My site uses mod_rewrite so I can’t check $_GET for the existance vars, which would seem to be an easy way to determine if a single post is being viewed.

As an example, when a single post is displayed, if the user is an “Author” role then I want to show a particular message. I can hook into the_content with the add_filter function, but I don’t see a way to hook into showing a single post.