How to detect what triggered the onbeforeunload event?

Hi,
I need to warn the user when he clicks on the close button of an aspx page.
And I'm using onbeforeunload event for that.
But I don't want to show this message when user changes the compatability view. Changing the compatability view causes the page reload. And it triggers the onbeforeunload event. Is there a way to detect what caused the onbeforeunload event or avoid the message box?
Thanks