Plugin for input Validation: How do I message the User?

I've written a plugin that does basic syntax checking on a page, template, or chunk in an effort to help identify problems like missing brackets, backticks, etc... I'm able to test the content and log an error message by tying into the OnBeforeDocFormSave and similar events.

So my question is how do I message the user from the plugin? Do I need to fire off a JavaScript alert? Can I return "false" and register an error message that users will see? And will this prevent the page from being saved?

Thanks!