How to detect when user clicks to install the webplayer?

Hello,

Using html/javascript, how do I detect when the user clicks on the little button to install the webplayer? I'd like to be able to do this so that I can track detailed metrics about how many people had Unity installed, of those that don't how many clicked the install button, and of those that clicked it how many completed the installation and get into the game. I know how to track the first metric and the last metric, but not how many people clicked the button.

Thanks!