Event handling to detect if user clicks a 'Like' button in XFBML page

I have a Facebook connect app / canvas IFrame app which has a Like control for a fanpage.

I'd like to detect when the user clicks the Like button to be able to display additional info (a discount coupon code).

I'm using the latest Javascript API but the events that can be handled doesn't seem to include detection of social widget controls. The documentation for the Like Plugin doesn't show any events.

However I know its definitely possible because if I have two Like buttons on a page they both instantly update each other if I like or unlike whatever it is.

Edit: It is specifically allowed to require someone to become a fan for a promotion :

4.2 In the rules of the promotion, or otherwise, you will not condition entry to the promotion upon taking any action on Facebook, for example, updating a status, posting on a profile or Page, or开发者_StackOverflow中文版 uploading a photo.

You may, however, condition entry to the promotion upon becoming a fan of a Page.

I think Facebook updated their documentation since I first looked :

Now its clearer :

http://developers.facebook.com/docs/reference/javascript/FB.Event.subscribe

A 'like' click is the edge.create event

Official FAQ:

Q: How do I know when a user clicks a Like button?

A: If you are using the XFBML version of the button, you can subscribe to the 'edge.create' event through FB.Event.subscribe.

Source: Core Concepts › Social Plugins › Like Button