I simply want to catch a "zoom" event and respond to it (similar to window.onresize event).
There's no way to actively detect if there's a zoom. I found a good entry here on how you can attempt to implement it.
You could also do it using the tools of the above post. The problem is you're more or less making educated guesses on whether or not the page has zoomed. This will work better in some browsers than other.
There's no way to tell if the page is zoomed if they load your page while zoomed.